Fingers, 100% of the time.
Picks are only faster if you have no control or finesse with your hands. If thats the case, why are you worrying about playing fast?
Using your hands is much more versatile, and thats NOT an opinion! You could play with your fingertip, fingernails, sides, or just the skin on the front (sorry, didnt have a better word for it,

). Not to mention then you can tap, slap, strum, etc, and you dont have to palm the pick while doing it.
And its much easier to get faster speed with fingers. At most, on a pick, you can go up and down, similiar to playing with two fingers, but with hands, you can play with 2, 3, 4 or even 5 fingers. Thats like playing with 2 or 2½ picks
simultaneously.
And this doesnt even get into range of dynamics, triplets (a bitch with a pick), etc.
Green Day, Nirvana, Blink 182...all have the best bass players? Wow, gosh, what was I thinking. Id like to see ANY of them play fretless like Steve Bailey, or make an interesting solo! But wait, theyre the best, and they play picks (therefore, picks _ARE_ the only option, right?)! VERY few of the highly regarded players play with picks, proportionally. Yeah, of course theres Jackon, Kaye, Squire, Osborne, Vega, etc, but they are a minority. Mark Hoppus...bass god? I think ill puke.