The most optimal tradeoff for Critical Chance and Critical Damage is on your weapon.
You can get 2 critical chance for every 3.5 critical damage you give up in leather offense. (this is 1:1.75)
I heavily do not recommend investing in cunning if you can help it.
For non rogues this is a 1 critical chance for 2 crit damage trade off.
For rogues this is a 1 critical chance for 2 critical damage and 1 attack % trade off. You should try to avoid this whenever possible. This is THE worst trade you can make for critical chance.
For rings you're also making a 1 critical chance for 2 critical damage, it's not as bad as above, but you should try to avoid it.
This assumes that you have an unlimited supply of ice dragon hide however, which not everyone is willing or able to do. Without unlimited supply, the worst trade off is 1 critical chance for 2 critical damage. Basically it means that the ring's aren't subpar tradeoffs, and neither is cunning on non rogues.