I used to buy that miracle grow moisture control stuff... I forget the nute ratio but its something very very low good for young plants. Its got peat and perlite mixed in if I remember correctly.
I recently bought the cheapest stuff they had and a bag of perlite and am gonna try mixing my own.
I believe they have a few different kinds of schultz brand soils too.
From what I know your best bet is to get something with perlite in it for good drainage, and something that doesn't have fertilizer or slow release fertilizer in it so you can have more control over how much nutrients your plants are getting.
Then again Ive heard a few people have good results with soils that come with fertilizer.
Personally I like to play it safe :]
Hopefully someone else can give you a more definitive answer