Damage is taken as a combination of stamina (or magicka) and weapon damage (or spell power). Each adds half to the equation, so an increase of 3% stamina would mean an increase of about 1.5% damage.
Critical damage is always 50% extra on top of your normal attack damage. On a crit, you'll deal 150% damage as opposed to 100% (unless you've got other bonuses to critical damage, which change the resulting value of the critical rating attribute). That means that each 1% crit chance will come out to 0.5% extra damage. So, 6% crit = +3% damage.

That's a heavily simplified way of calculating it, and there are other factors to consider, but in general a 3% stamina increase will boost your damage by 1.5%, while a 6% critical chance increase will boost your damage by 3%.