For tournaments I prefer 9-handed tables. I dislike 6-max very much in tournaments.
However, in cash games, I dislike 9-handed tables very much. And as you might expect, I love 6-max for cash.
Why? 6-max cash is less boring than 9-handed is. When we are in early position at 9-handed, we only really play the top 5% of hands, JJ+ and the 16 AK combos. That's boring folding 95% of hands in the 3 early position seats. We're folding 95% of the time in 33% of the seats. Yuck.
6-max cash is terrific. Even the earliest position is a very playable seat, we're certainly not just playing 5% of hands, way more than that. So in 6-max cash games, there's good action from every seat at the table.
For tournaments, I just don't like 6-max because (a) we're paying the blinds much quicker than at 9-handed tables, and in tournaments I prefer to remain tight and patient... so I don't mind folding all my hands from the 3 early position seats in tournaments. I know I'm gonna be there for 10 hours in the tournament (because I always plan to win!), so I'm well prepared for "boring" poker most of the day. 6-max is just too much action for my liking, as odd as that may sound.
Hope that helps.