My two cents.
The big difference between the two games is that you're paying blinds more often in 6-max. This is offset by the fact that you are in position more often, which means you can play more pots. So there is more action in 6-max.
6-max tends to be a lot faster, and has a lot more action. And action is where you learn the game. It's sort of a concentrated version of poker. It's like they boil off the 1/3 of the 9-player table that are in the late "should only play great hand" seats.
You fold a lot more in full ring, which is a bit tedious.
People compensate for the slower paced 9-player games by multi-tabling. I'd also assume that with more players, there's a better chance of having more fish at the table (though at 6-max, you get to tangle with the fish more often, and with less competition).
Basically, pick what you enjoy.
Whichever you do play, table selection is vital
. Look for tables where people are too active. I arrange them by the percentage of players that see the flop, and sit on the waiting lists for the loosest games. Basically, you want people who are willing to call you down when you have the best hand and chase when they have improper odds
to do so. So while I prefer 6-max, if I see a juicy 9 player game I'll sit there instead.