Which game ??????????????

Assuming you like both genres equally:

Halo: Combat Evolved (Halo 1) is the clear choice when
it comes to multiplayer. Though the community has obviously
dwindled down with the release of other games, it does still
provide a fairly solid multiplayer experience. I still consider
it one of the best as far as weapon balance and reaction goes.

Warcraft 3 (and The Frozen Throne?) offers a pretty fun single player
experience. Though not quite good enough to be referred to as "great",
it is definitely fun. I can say that I enjoyed this single player over
Halo's, and prefer Halo's multiplayer to Warcraft's. Warcraft's multiplayer
is very repetative, and always involves massing two units and nothing more.
Very boring

Finally: I'm a big fan of FPS Game & my advice to you is to Buy Halo: Combat Evolved But At the End you're the one who is going to choice..

Good luck..