If you're new to the game, do not try OD&D or AD&D.
-Huge amount of resources available, both in official content, third party content, and homebrew.
-Extremely flexible, if you like having an extreme variety of options to customize your world with.
-If you want to buy your books, you can do so on the cheap, (I recommend torrenting, however)
-No longer a supported system.
-Without supplements/houseruling, spellcasters outpace melee users dramatically. (Tome of Battle somewhat fixes this, inb4 Book of Weaboo Fightan Majik)
-Lots of specific rules make the game a bit tedious to learn, and have some confusing rules.
-Currently supported edition by Wizards of the Coast, which produces the game.
-All characters have a useful role in the party
-Easy to pick up and learn.
-Lacks the flavor and variety of third edition.
-Combat takes forever once you get into higher levels
-Classes, while streamlined and all useful, are very homogeneous and arguably boring.
-Poor background information, but if you like being creative that could be conceived as a pro.
Pathfinder (Bring on the trollan)
-Backwards compatible with 3e
-Streamlines some flaws in the game
-While melee has been improved, spellcasters are still very, very strong
-Prepare to be trolled for talking about it
-Relatively new, so the "update" process between Pathfinder and 3e isn't as easy as Paizo makes it seem, although it is still reliable.
I've played all three, and it really depends on what kind of group you want. If you want a relatively easy, boardgame experience, go with 4e. If you'd prefer more depth, go with Pathfinder and torrent the 3e stuff, pick and choose as you grow more experienced as a DM.