You should play in some freerolls
or hit up the play money tables for a bit and see which interface you like better. I like FT better, because the rakeback I get far outweighs the VIP promotions that pokerstars offers. If you're playing at 4NL i'm not sure that rakeback really matters, but then again I don't know if you'll be able to qualify for any of the pokerstars VIP stuff either?
Oh and 1 more thing to check, most rakeback sites require that you rake in a certain amount of money within your first 30-60 days in order to keep your rakeback account tagged to the site. I haven't really looked into this at all, but its something you might want to check on before you just assume that you have FT rakeback.
If you're depositing a decent chunk of change and haven't deposited on FT before I would DEFINATELY deposit on FT and clear the bonus....and then if you don't like to FT cash out and take that money over to PS. PS doesn't offer (that I know of) any deposit bonuses while FT offers 100% up to $600.
I've never deposited money on PS, so I'm biased.