3.5 for the win.
4.0 reminds me too much of WoW, everything is bunched together... Good for beginners I guess :P
Well yeah. My view is that 4.0 is easier to learn, but for those of us who have been playing since earlier versions, it's not quite as "classic" as 3.5 is.
I still can play both, though the only 4.0 books I have are the monster manual I and the Player's Handbook.
For 3.5 I have:
Player's Handbook
Dungeon Master's Guide
Monster Manual
Monster Manual II
Monster Manual III
Planar Handbook
Manual of the Planes
Draconimicon
Races of the Dragon
Heroes of Battle
Complete Arcane
Complete Adventurer
Complete Warrior
Complete Divine
Player's Handbook II
Spell Compendium
Magic Item Compendium
Deities and Demigods
Epic Level Handbook
Psionics Handbook