Flat fingers will probably be better for you in the long run on bass in my opinion.
It can seem easier to play fast licks with curled fingers, because the movement for each finger is the same. With straighter fingers your index and pinky are straight while the middle two are bent a bit. However, its easier to put more weight behind straighter fingers and requires less physical effort, which more than makes up for this to me. Plus, keeping strings you dont want muted is very important, and flatter fingers makes this much easier.
Do what Todd and Lemur say and you'll get it
One tip - Once you've got the wrist sorted out, try this: Lay your fingers on the E string. Keep your thumb where it is, and pull your elbow up, and you'll see that your fingers will move across the strings without you moving a muscle in your hand. The less work each individual muscle has to do, the faster you'll be able to play, and with less effort. That goes for all techniques
