Well if your planing to play up to 15$ sngs i played them last year on stars and those low stakes games aren't so reg infested.
You also have to consider that Stars have way better software then any other poker room, you can get table ninja which makes them way easier to multitable and you get good traffic at all times.
The rake is also way lower compared to Party and they have a better VIP system as well which more then makes up for 1-2 extra regs on the table. Not to mention the shitty software on party and constant disconect problems are a sure way to put you on tilt.
You can also table select a bit by not registering in games where there's already 3 regs or more waiting so you can easily get tables with no more then 3 regs which is still a very profitable situasion for you, especially considering that those regs are not great players anyway.
I never played sng's on any of the smaller poker rooms but im guessing its going to be a problem getting in the volume since the traffic seems pretty low. Those smaller sites are good mainly for getting soft competion in HU. You will probably have to play on more then 1 poker room at the same time to get in enough volume but the games are probably a bit softer then on stars.