Mm. I dunno.
The rules part of the revised set is probably better, in some ways. Tables are fixed, things are evened out a bit. However, I think the earlier edition did defiling magic better, and it included templar as a character class, which I think is the way to go. Personally, if I were running a 2e game, I'd use both rule books, and pick and choose what fits your preferences best.
As for everything else.... the revised setting gives us a history of the world that is a bit bizarre, and focuses on "life-shaped" items and the like. It posits a world that was originally run by halflings, and talks about ancient genoicidal wars led by the sorcerer kings. Unlike the original setting, which suggests that the current sorcerer-kings are just wizards of great power, and that there were more than likely other city-states beyond the Tyr region, the revised setting says "these sorcerer kings are the only SKs, and there will never be any more".
The revised setting gives a much larger area to explore (they say about eight times larger, but a lot of that is "empty space"). However, it adds a lot of silly stuff that feels very "anti-Dark Sun" to me. The plain of obsidian is an example - it's filled with undead gnomes and the like... and of course, since obsidian is so valuable, it's a lot like a plain of silver in any other campaign setting.
Then we have an empire of thri-kreen, civilized halflings that live on a cliff and ride hot-air balloons, and evil psionic overlords around a dying lake. Note that none of those elements are exactly "lame", and I could see a campaign built around any of them. But, for some reason, they never seemed very "athasian" to me.
The revised setting also gives the PCs a theme. Whereas the original set kind of just detailed a world, and said "go adventure", by the time the revised setting came out, it was assumed the PCs would be fighting against defilers, slavery, and the evils of the world. Some would really jump on that, and the 3.5E revision would put the PCs in the role of "underground revolutionaries" in a lot of ways. I never really bought into it, though - I liked a darker setting (and most of my players did, too). I guess TSR wanted to make the PCs good guys, fitting with their standard operating procedures at the time.
there is some very cool stuff from the revised setting, though. The bandit states. the city-state of Celik. Aarokocra as a character race. The trader class. And a few others I can't think of at the moment, but I'm sure are there.
Ultimately, I'd say get them both. Use the original setting as your "core", and then pick and choose what you like from the revised. It's a bit more work, but you'll have a better end result.