Calculating max critical dmg on a Hero

The question is whether 5% more crits offset the 1 attack difference.
Assuming a Hero with a total of 250 w.att, 1 attack adds 0.4% DPM.

If a Hero's crits are, say, 5% higher damage than her non-crits, 5% crit adds 0.25% DPM, which is not as good.
But if crits are 10% higher damage, those 5% add 0.5% DPM, which is better.

So, which is it?

If the attack from the cruel is more damage than the crit then i won't be wearing either ring as i have a lilins and both evo rings. So several of the upcoming rings will flat out beat the cruel ring for a spot. If the crits are only 10% higher than non-crits i probably will pass off the ring to my aran which would, if the OP is correct and Shinku is mistaken, get significantly more dpm out of the ring. In short in my circumstance it is wholly about the crit being significant on its own. I can do better than the other stats on the ring.