Depends...
I would say, if you don't count then it is generally better to play at fuller tables since you will get less hands and hence your money will last longer. Also, this way you will get a better comp rating.
For counters, it's more complicated I think. One-on-one gets you more hands per hour which theoretically should increase your hourly win rate. In single/double deck games, less players is usually always more desirable since you will be more assured of picking up those high cards in positive counts (this is also true in multiple decks). However, in shoe games especially I find you can get stuck into really negative shoes; then it's handy to have others at the table absorbing up some high cards. Fuller tables also seem to serve as a good cover, since most non-counters wouldn't dare play one-on-one with the dealer, and it is also a good cover for wonging-in and out. I prefer playing shoe games with 3-4 others, so if the count does get very favourable there is room to also play two hands. Hope that helps a little.