Unlike crit dmg calculation, I haven’t looked into gun damage calculation yet in BL3, but from all I see it seems to be similar to BL2. The main point is that crit dmg is based on gun dmg, so in a simple choice of +50% gun dmg vs. +50% crit dmg without any other bonuses gun dmg clearly wins because the end result is the same on crits but lower on regular hits:
With gun dmg +50%:
Regular hit: 1000 x 150% = 1500
Critical hit: 1000 x 150% x 2 = 3000
With crit dmg +50%:
Regular hit: 1000 x 100% = 1000
Critical hit: 1000 x 2 x 150% = 3000
Now this can change to some extent based on all the other skills and gear and stuff going on, but the general tendency remains. With lots of other additive crit bonuses, +50% gun dmg may even lead to a higher increase on crits than +50% crit bonus.
In BL2 there were only a few situations where crit dmg applied but gun dmg didn’t, like melee attacks. But in BL3 melee can’t crit so that one’s out of the window already.
So there may be exceptions but as a general rule I’d choose gun dmg over crit dmg if they’re the same value, and much more so if gun dmg has a higher value.
edit: FL4K’s Fade Away is a special case where crit dmg does nothing, as it’s based on gun dmg but generally unaffected by crit modifiers. You can increase Fade Away damage with more gun dmg, but you can’t increase it with more crit dmg.