[~SHWING~]
Picks are nice and a great way to start but your fingers can comp. the "twang" with proper effects.
With a "walk" (the index and middle fingers going back and forth) you may easly get the speed with practice.
If you want you can also slap Van Wooten style with a ring that is between a pick and a slap.
Personaly I feel picks limit the sound range in all types of music, metal is no excception. (Iron Maiden any one?) There are lots of pros out there who play with picks because of the sound they produce. It's much sharper than playing with your fingers. Trying to emulate it with effects is pretty much useless, since you can achieve the same sound through buying a $0.20 pick and simply practicing to play with it. Plus, I don't know which effects emulate that sound.
Slapping like Victor Wooten does not sound different from anyone else's style of slapping. He just controls his slapping so that it still sounds good, unlike lots of people who think slapping is simply knocking out your strings with your thumb...
And I don't think picks limit your sound. Not learning to play both ways (pick vs. fingers) is limiting yourself. As both sound completely different, there's no telling which of the two is less versatile.