I can't speak for JC2, I haven't played it. But I had good fun with AVP. The campaigns were... ok. Fun in parts, especially as the Alien.
Multiplayer I think is great, it's, like the campaign ok, but now either really great or really shit. If it's in a three-way deathmatch, it gets seriously tense when you're being hunted as a Marine, and it's great fun stalking prey as the extra-terrestrials.
However, sometimes there'll be a case of "HOW THA FUCK DID HE KILL ME!!!?!". You know, when you're pretty damn certain the kill is yours, then he turns around and skull fucks you in the face. That can get pretty annoying.
I wonder if there are any people still playing the MP, though...