This is a hard question to answer. Just so you know, I sold all my items that aren't lvl 50. If you are not at max level yet, compare the gun to similar leveled items. I remember having a blue rocket launcher on my first playthrough that had 17k dmg at lvl 25 or something, which was miles above anything else at the time. If you are lvl 50, the only things worth keeping are guns that improve your overall dps, and any legendary(orange) is nice to hold on to. General guide, white < green < blue< purple < orange. There are seraph(pink) items from the toughest bosses, but I only find the shield and artifact useful. And each weapon has different things you want to look for, like SMGs need high rate of fire, and shotguns should have high projectile count. And every gun needs a good reload speed. Play around with the guns to see what suits you best.
You generally want a fast recharge rate and delay on a shield, until you get to the seraph artifact where the amount of hp you have is more important. My shield adds lots of hp and gives me hp regen. Combined with my artifact that gives hp regen, it makes me a very tanky.
This post was edited by Flogger101 on Jan 4 2013 05:01am