i switch between pick, fingers and slap all the time, sometimes using all 3 in one song. it depends on the sound you want.
for example, if you want a "hard", "flat" sound, then go for a pick. we've got a few songs where the best sound for the bass is a full-on, very rough, very hard, very flat sound. it's in your face, it's up there with the guitar, and it gives a harder atmosphere to the song.
we have other songs which are kind of darker, more harmonic, and i'll play with fingers. you can get a "fuller," "deeper" sound with fingers because the meat of the fingers is brushing off the strings instead of a plastic pick. even fingernails will give you a slightly different sound. i use this in some of our slower songs and our more harmonic songs where the bass should sound a bit "deeper" and "darker". it gives you a different emotion, a different feel to the song.
and lastly, i slap during parts of other songs for an entirely different feel. it's not so much upbeat as it is just more of a percussive feel. i play a 5 string, so on some songs, i drop down to the low b and start slapping my thumb on it over and over again (kinda like korn but not so poppy, it's still on the rythym pickup so it gives a deep thumpy sound) and adds to the percussion while still having its own unique bass sound.
it's all personal preference, but the best way to do it is to be able to play all of them. that way you can switch for everything you want to do.
i also use a pick for scratching on the strings, too. not all that often, but you can get a neat scratching bass sound sliding the pick around near the bridge. it's hard to hear without an enhancer pedal / effect or at least a really nice amp.