It depends on the type of game.
I have great success in MTT's, so obviously I am pretty good at playing tight and succeeding at 10 person tables.
When it comes to SnG's and ring games, I prefer 6 man tables. Playing short handed gives you a lot more opportunity to be aggressive and
bluff your way into winning a pot. Ring games, I profit much more at the 6 person tables.
I have a special place in my heart for 6 person tables because I learned how to bluff and read my opponents at these tables, which obviously improved my game greatly.