Personally, I don't think either choice is wrong. However, arm yourself with this info:
RE5 has a "stand to shoot" philosophy. In other words see those bad guys? You have to stop to shoot at them. This is the single largest complaint people have about the game. They want to run and gun and that's just not the RE style (yet). Try the demo to see if you can tolerate it. Yes, it has online and offline co-op but no vs. to speak of.
L4D has an amazing co-op system and a great vs. gameplay. Whereas most games you and your opponents are the same character just with different skin, L4D puts you in the shoes of the special infected which is a lot of fun. Solo is good enough to warrant a play through now and again, even though I've been through them all several times. It has no real story, but gets by just fine without it.
I hope that covers the basics you are looking for and helps.
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We're Oscar Mike!