what's ur current stats?
Current prep/real gmat score
Care about the price?
If 50 pt is all you need i think knewton might be better with their guarantee. If not it is up to you. I took knewton and liked it and there are others who took mgmat and liked it as well. I dont think there are many/any one here who took both to really give a solid comparison though.
If you like my answers please +1 kudos!