Let's talk about Attack, Critical Hits and Critical Damage.
Attack is the easiest to understand. If I tap once, and my Sketchy pokes Beowulf in his chub, I do 100 points of damage for that attack. Your Sketchy may be different, but that is (for the sake of this demonstration) what my Sketchy does.
If I add a move that gives me +6% Atk, then Sketchy will poke Beowulf for 106 points of damage. If I add +50% Atk, then I do 150 damage.
Add to ATK%, you add to your damage for every attack you land. Easy. I am not talking about comparative defense, blocking, or anything related to that.
On to Critical strikes. At their worst, fighters start with a 5% Critical Hit chance, and +20% Critical Damage. If Sketchy is lucky, his 100-point attack will actually levy 120 points of damage on poor Beowulf. (Frankly, he had it coming)
Of course, we are not playing for first touch, we are playing to beat Beowulf into unconsciousness. We will be attacking often, and landing scores of hits. On average (and we will not be average that often, to be fair), we will hit Beowulf with a critical hit once every twenty strikes. That hit will be 120 points of damage, and the other nineteen will be only 100 points of damage.
If we sum up those twenty hits, we have done 2020 points of damage. If we average that out, each hit will be "worth" 101 points of damage (right? 2020/20 = 101)
Hm. Our 5% crit rate and our 20% crit damage... 5 x 20 % = 100% (Per cent - per 100). divide that by 100, and you get 1. 1 is also the extra damage that we have dealt (on average). This is not a coincidence.
If we have a 50% crit rate, and a +120% crit damage... 50 x 120 = 6000, divide by 100 = 60. half the time we do 100 damage, and the other half we do 220 so 10 x 100 + 10 x 220 = 3200. Divide by 20...160.
Yep, it checks out.
Your expected damage boost for Critical strikes is as follows: Expected boost/hit = (Rate * Damage / 100)
But, your expected boost for adding to your Attack? = Attack. The math is much easier here. Attack is a direct single boost to your damage, but Critical strikes require you to boost both rate and damage. Going back to our basic Sketchy example:
Sketchy has a 5% crit rate, and +20% crit damage. If we raise our crit rate by 6%, we go from 101 damage to...
11 x 20 / 100 = 2.2 bonus damage. 102.2 damage.
We boost our damage by one more point to 102.
Critical strikes are a nice bonus, but one should strive to boost Attack. If you have a character that triggers off of a critical strike, then pay attention to your Crit Rate. If you have a character that has a Crit Rate at or near 100%, then boosting Crit Damage is effectively like boosting Attack. Otherwise, boost your Attack. Always boost Attack.