6 Months ago the general rule of thumb is if you're under 1GH, stay with Ethermine.org.  Today since the difficulty has double, I would say the rule of thumb is 2GH or higher.  Etheremine.org splits all Uncle payments among the entire pool so if you want guarantee pay, stay there.  If you want to GAMBLE go to Ethpool.  The bigger your hash, the more likely you're going to get an Uncle and keep it.  But if your luck is bad and you're not hitting uncles often, Ethermine would be more pay.  If you pool hop often to other coins, stick with Ethermine as your credit won't be worth anything until you have enough to hit a Block which right now is over 155T and quickly growing!