I like both, for different reasons.

Full-ring teaches patience, knowing when to duck a second best hand.

6-max teaches aggression, lots and lots of betting, Ahghghhg.... raisitis, is a fun disease to have.

For pure profit I play mostly full ring because it doesn't demand the concentration of 6-max, however, get a 2-3 strong coffees in me, and I'm wired for sound, bring on the 6-max. If you are at all distracted or tilting, 6-max is a good way to flush your BR.

Comparing the two is like comparing apples and oranges, It is better to be good at both, because poker demands the ability to adapt quickly to situational changes.

I find changing gears to be more effective in full ring, where in 6-max you basically keep playing aggressively or you'll be killed, by stinky primates that like to eat banana avatars.