Both Starfighter and Jedi Starfighter are fun games, but if the game's length is important to you, then you'll want to rent only. Both are very short and it doesn't take long to unlock the various bonuses either.
Lots of fun, though. Definitely worth checking out.
I beat Jedi Starfighter in 1 5day rental. It was a lot of fun. You should be able to find a used copy for $20 or so, so if you have someone to play co-op with I'd say go for it