They are drastically different...
There is no single player in TF2, it is a strictly online multiplayer game. And COD4's single player is pretty awesome.
That said, if you can only have one FPS...it's a hard choice. I would honestly take TF2 over COD4, because while COD4 has single player and the multiplayer is rock solid, I find TF2 to be much more addicting, fun with friends (as well as random players, people actually chat and have fun, given the style of the game), and well balanced.
Then again, if it's a hard choice, consider that TF2 is 7 dollars here compared to COD4 at 50.
edit: @RubangB, and Half Life 2, HL2: Ep1, and HL2: Ep2, which are considered the real "single player" games in the OB.
LEFT4DEAD411.COM
Bet with disolitude: Left4Dead will have a higher Metacritic rating than Project Origin, 3 months after the second game's release. (hasn't been 3 months but it looks like I won :-p )
