Depends what you're using it for, although the autogun lines are both considered pretty poor choices.

If you want to go into FSC with it go the Needle path, because the piercing damage would be good for killing trojans and vanaduke, both of which the immobility of an autogun wouldn't be as big of an issue, and they are large enough targets that the spread fire should mostly hit. The peppergun wouldn't be very useful there because most enemies resist fire. (and the ones that don't are weak to piercing) this is generally assuming you run a primary gunslinger setup, since if this is your only utility gun then you'd be in trouble taking out gun puppies, and using it as your only kiting weapon would be kind if useless since you have to stand still to fire.

If you want to use it as a general purpose weapon (i.e. you mainly use swords or bombs but feel the need to have a gun around) then the pepperbox is better, since enemies weak to piercing dodge like crazy and you're probably better off using your sword. I'd recommend going Pulsar (either line) or blaster or even an elemental alchemer over the pepperbox as a standalone utility gun though.

TLDR: Blitz Needle if you're a gunslinger wanting something different in FSC, Pepperbox if you're someone else who just REALLY likes autoguns.