RE5 for sure, it is the most Resident Evil has the most classic zombie games, it's original and they were the first zombie games ever made. Plus the new co-op option will make the game 5x better, Resident Evil was allready good before but now it's going to be awesome. Also for Left4Dead the hunters, tanks and witchs dont make sense in a zombie game, Resident Evil can do that super zombie stuff because that's part of the story line, DNA testing and manipulation.
-Great game, and it's a lot of fun with freinds. -Online is okey too! -Fun factor vary high -Many call it the "Best Zombie game so far"
~Con's~
-Get's old after awhile, and not something you can play for months -No story/plotline to it -Can get boring after you beat it
RE5
I can't say, beacuse it's not out yet.
Thoughts.... well it looks good, but many claim it to be the same as Resident Evil 4, some of the featuers are from RE4, such as the "zombie flinch". (When they get hit by bullets) Aagin, movements LOOK the same, and act the same. Baseically RE4 but a bit more "amped" up.
There is also really good feedback too! People/critics say it fun and vary good.
I have L4D, anyway first you should try em' both out, rent em' then descide if one of them is the one for you. If you need to buy, then wait for RE5.