Which critical ring is better for me?

I just go with logic..
I see 3m critical hits and 2.2 normal hits (or 4m and 3.1).
that difference is so big that I don't think even a 40% int ring will get the same effect of having 100% critical.

The math man can give more exact numbers, but in a quick dash through, you aren't actually comparing 3M v. 2.2M damage, that would be comparing 100% Critical v. 0% Critical. The more accurate comparison would be 3M v. 2.88M comparing 100% v. 85% Critical ((0.85*3M)+(0.15*2.2)=2.88M). That's some of the basis behind hadriel's suggestions regarding the %INT, etc. That's if there are too many numbers to digest there.

EDIT: Now that I've been ninja'd, I recall either hadriel or Grey having a table with relevant critical, maybe minimum critical data. I don't remember where it is now though.