I tend to buy more wizards stuff, for several reasons:
1) Balance: Wizards is far from perfect, but most d20 stuff just seems like it isn't balanced to me. With this I'm only talking about rules issues, not adventures and such.
2) I don't run pre-written adventures often enough to buy them with all the free one's online.
3) "Alternate" Rules and versions of classes don't usually impress me. Sorry, Shamans, Chaos Magic, etc.
4) Mechanics for the sake of mechanics:
Hear me out on this one. The primary culprit here is prestige classes. Something that, conceptually, is already in the game (more specifically, the core rules), is introduced with another mechanic. Examples: any archer prestige class (if you want to be good with a bow, there are plenty of feats in the core rules), fighting style rules (also covered by proper feat selection), and other nonsense prestige classes like the Shadowy Avenger or the Ghostwalker (it's called roleplaying, you don't need mechanics).
For an example of a book with alternate classes and prestige classes that are nothing but these things, look at the Quintessential Rogue. Everything in Character Concepts and the Prestige Rogue can be achieved with the core rules with proper feat and skill selection and role playing.
Also, almost all of the feats in Beyond Monks fell into this category for me.
Again, Wizards isn't without blemish here. In fact, most of the splat book prestige classes are just this.
I will say this: I understand the need for more fine tuned differences in an all-fighter game or something (like Rokugan does with samurai), but for the broader games I run there's no reason for this many options that accomplish the same thing conceptually. I guess you could call me a rules minimalist.
5) Information about what's in Wizard's stuff is more readily available.
6) Superior follow up- Wizards might not be the best on errata, but they strive to make sure that future supplements are compatible with present supplements, whereas most d20 rules supplements are one-shots, never menitoned again.
7) Power Creep- It's inevitable, and Wizards suffers from it, but buying mostly them limits it.
8) The Forgotten Realms and Star Wars are my favorite settings.
For the record, here are the d20 products I own:
The good:
BOEM I & II (but some stuff that I don't like, such as feats that are powerful but are "offset" by requiring high ability scores, so characters who are already powerful can get MORE powerful)
Quintessential Rogue (alternate uses for skills and trap construction on the fly rules are both superb)
Traps and Treachery (best d20 supplement out there, IMHO, for chapters 3-5)
The Giant's Skull- a really original adventure idea, it was fun for a one-shot
Counter Collections I & II (Ok, maybe the only thing better than Traps and Treachery)
Rokugan- Courtier class is superb, other core classes are blah, lots of mechanics that are only good in a game with lots of samurai that REALLY need to be differentiated- everything a setting sourcebook should be
The So-So:
Relics and Rituals- bad prestige classes, poorly done magical items, but some pretty good spells
The bad:
Swashbuckling Adventures- Takes the mechanics for mechanics sake idea and turns it into a dogmatic religion. This had way too many classes in it. It might be good in a Rokugan sort of way where minutia matter since the genre is so fixed, but I consider it bad because basically nothing from this book is portable into my games. Also, it doesn't maintain ANY sense of internal balance.
Beyond Monks- The Martial Artist class had some potential, but fell on its face. Finishing Move? That's something a 13 year old would write. The feats, as I've already said, are just mechanics for the sake of mechanics. The only thing I liked was the feat that lets a monster pick up a smaller creature and club people with it.
