Impact is the amount of damage done per shot, as well as the amount of damage multiplied when hitting critical. When looking at any 2 weapons with the same level, the weapon with higher impact will do more damage per shot than the other. Attack is a modifier of impact. It takes the base damage of any weapon type And multiplies it to get the actual damage dealt. 2 weapons with the same attack will not always do the same damage as they are modifying different impacts. It is also required to have weapons with a certain amount of attack to harm higher level enemies (remember your first time In the grottos when you couldn't even touch the lvl 20s? That's why). Basically, you want to look for the weapons with the highest attack. And then you want to go for higher impact for faster kills (tho you should still be aware of weapon type, mag size and fire rate). Also take into account that your level is also a big factor in determining your damage output.