So it seems like people have a very basic understanding of these two stats, impact and attack. I thought I would toss in my 2 cents of what I found during my brief testing.
If you hover over the attack number the descprions actually says "higher attack allows your weapons to damage higher level opponents" and it seems like people have found that to be accurate.
Now what I was what if a gun with a lower attack had the same impact as a gun with higher attack, here is what Got:
level 10 hunter
Auto rifle with 60 attack at medium range against a lvl 6 Dregg: 28 dmg to body and 84 to head
Auto rifle with 72 attack and equal impact as the rifle above at medium range against a lvl 6 Dregg: 31 dmg to body and 91 to head.
So if the enemy is of equal or lower level and two weapons have the same impact, the one with higher attack ultimately does more damage. Now, I then switched to scout rifle wth a much higher impact but same attack as one of the rifles and here's what I got.
Scout Rifle: 72 attack with more than double the impact of the previously mentioned rifles: 56 dmg to body with 138 headshot damage.
So I guess in the end you really need to look at impact unless you have guns that are at parody, have elemental damage or are facing enemies of a larger level.