Hm, they're just so different shows, it's really hard to compare them. MLP more closely follows the formulas of children's cartoons, while Adventure Time is just sort of the creator's imagination running wild that just barely fits within the confines of children's television--or any television in general, to be honest. But both are great because they know exactly what they're about, and they have the perfect sets of characters to achieve that.
MLP has a large set of regular characters with very specific quirks and flaws which all balance against each other well, making it easy to follow their show's format. And Adventure Time has a small cast of regulars with a large cast of other random characters, that makes following the whims of the writers very easy.
So I'm going to be a stick in the mud and say both. Both have the ideal casts of characters needed for what their shows set out to accomplish, and both succeed brilliantly.