Axton is proving to be a great sniper for me, and I've haven't even invested anything into gunpowder yet. But it seems to me that I'd rather have higher raw damage than a higher critical multiplier, because snipers come with a critical bonus anyway. With +200% crit bonus, the 50-80% gun damage you get from Axton's gunpowder tree will get tripled and then some on a crit, easily meeting and exceeding the bonus crit damage Zero gets from investment into the sniping tree. Where Axton really pulls out in front is all the damage he'll get when he DOESN'T land the crit.
That's not to mention that with Axton, you have the option of swapping to another gun and still wrecking quite a bit of face.