ive only tried X and O, im using oring currently. Didnt really notice much if any difference in wear time or anything. Neither chain really stretched much on me. they'll all hold up if you take care of them. Clean em once in awhile and keep em lubed.
From what i uinderstand the X ring has two sealing points to keep the lube on the rollers as opposed to the O ring only having one sealing point...So inturn they are supposed to last longer...Never used the X ring so im not sure