For tournaments, definitely 10 players (live) or 9 players (online). As a TAG (tight-aggressive player) I find that 6-max tables require me to play more hands than I want to be playing, and even 8-handed doesn't seem to suit my style. I know tournaments are a marathon and not a sprint, which is why I mainly stick to playing the top 15% of hand values. Tight is right!
For cash games, I much prefer 6-max because it is incredibly boring to just play 15% of hands, as opposed to tournaments where my mindset is to have extreme patience due to the length of time it takes to win. So for cash, I prefer to open up my hand range really wide and play a LOT more hands and do a LOT more bluffing against certain player styles. So it becomes the opposite from tournaments - playing full ring tables with such a wide range is big trouble, but it suits 6-max format much better.
Hope this helps, just my opinions!