I'd say go with Timesplitters 2. I think it's come down a bit in price, and it's a really solid title. My friends don't like playing it as much as Halo when they come over to my place, but I think it's fun as hell. There are a lot more options than in most fps, and there is even a map builder function that is a lot of fun to play around with. Tons of stuff to unlock too by finishing different crazy missions. It might not feel as tight as Halo, but it's still a really fun title well worth your money.
"The Universe is shaped exactly like the Earth. You go straight long enough you end up where you were."
While it is no GoldenEye, Agent Under Fire is still a fun multiplayer shoot em up. My friends and I still go at it with that one as well as with Smash Brothers Melee. Yes, I know it isn't a first person shooter, but dear lord is that still a fantastic way to kill time with 3 other people.
PS2: turned 10 years old yesterday. See: list of developer's favorite games. It was supposed to last 10 years, and they were still putting on new games this week. (Not many, but not many for PSP...) $$$: