The 9-man $3.40s on PokerStars are regular speed, while the 10-man $3.40s are turbos. That's the main difference. Regular SNGs take longer, but give you more play in the profitable early levels, so slightly higher ROIs are achievable in regular speed tournaments, although many feel that turbos let solid winners get more $/hour.
One past advantage to playing $3.40 turbos was that it was the lowest level of turbo SNG (while there were $1.20 regular SNGs), but I see that there are $1.75 turbos now. Any time you can play the lowest stakes game of a type available, the games will be soft. This is true even if it is a live NL $500 game which is the smallest NL game at a casino.
10-player SNGs let experts get a slightly higher ROI than 9-player SNGs, although they take a little longer, they produce a slightly higher variance, and they require slightly more complicated decisions.