what gear ratio do you have in your differentials? do you do a lot of off-roading?
IMHO...If you go off-road often, go with the 31's (regardless of gear ratio, even 4.10's). If you spend all your time on the road, go with the 30's.
I have had my 30x9.50's for 5 years. Originally i had 4.10's till I got my e-locker and it had 4.30's. Either way, I felt that the 30's were fine, and would have rather had 31's.
However, since I now have other dedicated trail tires on their own rims (34x9.50 tsl's) I like the 30's for everyday street use.