Depends on your situation and UV's:

Callahan has knockdown on softbodied non-jelly creatures )gremlins, wolvers, chromas, devilites) on each hit, so IAS or damage are both useful (and I say damage because shoot-shield can keep a steady juggling rhythm). Its charge attack is rarely ever used.

Needle shot has a spray and spread and very long range (longer than the Callahan on its regular shot). Its regular shots do not knockdown monsters though. For the autogun line, the most important UV (at least to me) is CTR. Now, the needle shot line's charge attack keeps you completely still and forces reload, but it shoots about 15 bullets, each with increased damage, a fixed spread rate, and each one with the knockdown ability. It has insane damage and utility, with the main drawbacks being that it keeps you still (not a problem if you're juggling/killing your target) and that you have to wait a few seconds to charge.

Blitz is useful 1v1 and vs Vanaduke due to type bonus and that it's so easy to hit all the bullets; Callahan is better vs small mobs 1-3 where its invisible explosion can catch them.

Ah.

I was fortunate enough to get a needle shot with CTR:medium, so since charge attack knocks enemies down, I use that to kill wolvers. The main drawback is that even though it's pretty much a 1 hit kill if you get a good shot, you can only really do that to one monster at a time :x