In reguards to what you should buy, take a look at the rest of the system you're building. If you've got a 7870 planned for that bad boy, then an AMD chip will do you up just fine. Pair of 680s in SLI, you're gonna want the I7 most likely. How long do you plan to use this machine personally? Are you wanting the best or the best bang for the dollar? The prices on both companies chips are right in line with performance, give or take a few frames. Also important to looksee at the resolutions you're playing at or planning on playing at. 1920X1080, you probably don't need a 3570k. Blashpemy, I know. I have one with a 7870 XT I just built and I get 200FPS in some games, that being said, I don't notice that much of a difference from my old Phenom 965 BE + 5770, even though with that I was only getting around 80 FPS (Both instances are with tweaks to eye candy, the 7870 has more candy, but the game seems about the same), I'm in a unique spot though, I'm on a 40in LED monitor, so I'm not right up against the screen.
If you're building a system for total less than a grand, I'd greatly consider an AMD build, over a grand, there's no reason not to get a 2500k or 3570k.