Why Diane Duane's Young Wizards is Better Than the Harry Potter Series
1. Number of Books
YW: 9 novels and several short-story collections, with more of each currently being written.
HP: 7 novels, 3 tie-ins
Obviously, quantity doesn't beat quality, but it doesn't matter because there's about the same amount of material on both sides. It should be noted, however, that while only 7 of the HP books are about Harry and his friends, all of the YW books are about its main cast, with different amounts of emphasis on different members of that cast. So this one is entirely subjective.
2. How The Characters Learn Magic
YW: Wizards are apprenticed to older wizards in their area. Characters still go to school and have jobs and lives that are not explicitly magical, thus helping to hide the existence of wizards from the world.
HP: Wizards go to their country's wizarding boarding school. There they are instructed in magic, and little else--"Muggle Studies" is an elective, as is Arithmancy (the only math-related course mentioned in the entire series). Squibs (non-magical people born to wizards) do not receive any sort of useful education at all, because they only learn about the magic they cannot use. Wizards form their own parallel world with a high reliance on magic for everything; they hide their existence using memory spells, magical camouflage, and other such tricks only.
Young Wizards seems a bit more practical here, and also has wizards as part of the world around them instead of some sort of sectioned-off elite. Furthermore, they don't force any of their own to be pigeonholed into drudge work through a lack of useable education. So score one for Young Wizards.
3. Ethnic and Religious Inclusivity
YW: The main characters' religions aren't clearly mentioned; their families may not be religious at all. The human characters are racially diverse, with both Black and Latino characters as part of the main cast. Non-human characters do not appear to be based on any racial or ethnic stereotypes.
HP: The main cast is very white, as is the series' primary setting of Hogwarts. One or two Jewish characters are mentioned, but everyone celebrates the religious holiday of Christmas. There are a handful of minor characters of Asian descent, and one character is portrayed as Black in the movies (but changes to a white actor halfway through). No character is explicitly stated to be Black. Characters from other countries are clumsy, one-dimensional stereotypes, as are their schools (the extended lore at Pottermore is even worse about this, but I'm focusing on the books and movies here). Pureblood wizards are generally prejudiced against wizards born to non-magical folks. Goblins are blatant antisemitic caricatures, and this is even more apparent in the movies, where their big noses are rather prominent.
Young Wizards is the clear winner here.
4. Inclusivity of Disabled People
YW: While the only disability explicitly represented in the series is autism (and the newer edition of the book which introduces the autistic wizard is much better than the original edition in this regard), it is implied that all wizards with disabilities are treated with dignity and respect by their fellow-wizards, because Life takes many forms.
HP: No disabled wizards are even referred to. The closest thing we have to a disability in the HP universe are Squibs and werewolf Remus Lupin. Squibs, as mentioned before, end up doing drudge work regardless of whether or not they are happy with this, and often resent wizards for it. Lupin's lycanthropy is a rather clumsy AIDS metaphor which implies that there is nothing wrong with the fact that "People will not want a werewolf teaching their children." One wizard (Neville Longbottom) is portrayed during the early books as clumsy, and no one seems to be the least bit sympathetic. Instead, he is continually mocked.
No contest. Young Wizards wins again.
5. Depth of the Wizarding Community
YW: Not only are ordinary animals capable of being wizards, but there is an intergalactic community of wizards in the books. Wizards may be humanoid, an Earth species, or some wild new creature created for the books. These creatures tend to be very imaginative and not to follow the typical "everyone is a humanoid with random extra bits slapped on" aliens that sci-fi in TV has made unfortunately prominent, nor are they typical fantasy creatures. Wizards canonically believe in doing good for the community, wizard and non-wizard alike; there is implied to be a wide variety of cultures and mindsets, and there is always the tantalizing feeling that a culture is complete and well-thought-out even if we only see a bit of it.
HP: The Wizarding community consists of humans and of the more intelligent fantasy creatures (centaurs, for example). It is an insular, hidden community that has universally chosen to remain separate from non-magical folks. Everything in the Wizarding community runs on magic, from transportation to candy-making to communication, making it a distinct culture separate from everybody else; despite the availability of Muggle Studies courses, it is repeatedly demonstrated that Squibs and wizards do not and cannot fit in well in the non-wizarding world unless they were raised in it. Most of the creatures are adapted from centuries of folklore and magical tradition, but there are a few rather inventive creatures Rowling created herself. The unfortunate thing is that none of this is examined in any depth in the books or movies. JKR seems to have simply not thought about the implications of anything, to the point that in Pottermore, she states that prior to indoor plumbing, wizards did not use outhouses or guzunders but simply did their business anywhere, like animals, and disappeared it after.
I'm going with Young Wizards here, and not just because of the poop. Both authors create innovative new species, but only Diane Duane seems to actually do anything interesting with them.
6. How Magic Works
YW: Wizards speak in the universal language of life (which is implied to be rather similar to Irish Gaelic). They use this language to gently convince the universe to do what they want. Children tend to have more power than adults, but adults have more subtlety and finesse simply from experience. An apprentice is often reminded by their teacher to respect themselves and others, avoid trying to compel people to do anything, and basically to be careful not to accidentally blow themselves up. Some spells involve the casting of magic circles, which are only visible to wizards.
HP: Wizards tend to do a lot of waving of wands and saying things in pseudo-Latin. It is stated that wand movements and spell pronunciation must be exact, which creates the question of how on earth anyone ever discovered them in the first place. Some spells involve potions, which must be carefully measured and mixed together in a very precise order. While a spell to force people to do whatever you want is forbidden, a spell to force people to fall in love with you is treated as normal and harmless. Altering people's memories or forcibly changing their forms is also seen as no big deal. (Curiously, changing your own form using a Polyjuice Potion is forbidden.)
Young Wizards wins, both on grounds of consent, and on the grounds that her magic system could have conceivably evolved organically and been discovered in-universe through trial and error.
7. LGBT Inclusivity
YW: Sexual orientation of some characters is hinted at, but not explicitly stated. There are no canonically intersex or trans human characters as of this writing, but alien species often have more or fewer than the two genders that Western society tends to divide people into. None of this is treated as anything weird, nor are wizards sexist as a general rule; Life comes in many forms and that includes a variety of genders.
HP: No mention of sexual orientation, or even hints, in the entire series. Dumbledore was retroactively assigned a sexual orientation in Pottermore. There are occasionally bits of sexism. JKR herself has stated that she is proudly against trans and intersex people to the point of wanting them dead. She has also stated on multiple occasions that she considers all fans of the HP books and movies to also be in favor of killing off trans and intersex people.
YW doesn't even have any competition for this one.
8. Movie Adaptations
YW: None.
HP: All 7 main-series books have been made into movies, with an additional trilogy about the Grindelwald incident.
HP wins this one. One point for HP. Woo.
9. Authors' Other Works
YW: Diane Duane has written dozens of books outside the Young Wizards universe, including (but not limited to) a high-fantasy series, a fair number of officially-licensed Star Trek novels, and several animated movies. These works are loved by fans and have a small but dedicated following.
HP: J. K. Rowling has written very few books outside of the Harry Potter universe. They tend to be critically panned and are usually bought mainly by hardcore HP fans.
I have nothing against a first-time author, as JKR was when she wrote the first HP book. However, it's been 25+ years and she still doesn't seem to have branched out much. Duane may have been writing for far longer, but even comparing JKR's current bibliography to what Duane had written in the first quarter-century of her writing career is shocking. Quantity does not imply quality, but being able to write a variety of books can and does improve one's skills at writing one's favorites.
Plus, there's the fact that JKR frequently publicizes that all fans of her works want trans women to die and trans men to somehow become cis women. Diane Duane's Young Wizards series wins hands-down, 7 to 1.