Two years ago, this wouldn't even have been a discussion.
I give the creators of MLP:FiM (Lauren Faust and co.) all the credit in the world for stretching the premise of My Little Pony as far as they have... WAY beyond what I thought was possible, personally. I am amazed at how thoughtfully crafted the world and its characters are and how accessible and effective the humor is - and I enjoy the show for these reasons.
When I first heard about it, though, I thought it was destined to become the poster child for a new generation of half-assed, cheap, profit-driven, focus-group-derived corporate bullshit designed to brainwash children into associating television with overpriced novelty crap. I thought the show itself was going to be a series of thinly-veiled, candy-colored commercials created by the lowest bidder. I was certain that if it succeeded, it would do so at the expense of creator-driven content such as Adventure Time.
Much to my fascination and delight, however, MLP managed to transcend its commercial origins pretty much completely. Even if the franchise didn't exist, this show would stand on its own merits. It combines good characterization (arguably great - there are very few positive female characters in mainstream children's cartoons today) with extremely appealing design, excellent voice-acting, and very good writing (especially considering all the extra constraints placed on it by Hasbro, standards and practices, I/E, etc.) - not to mention the animation itself, which is superb by any means for a mass-produced television show and contains some of the best vector-based animation I've ever seen.
Having said that, Adventure Time gets my vote. For all of MLP's blessings, it doesn't quite stand up to the visual innovation and creative splendor of Adventure Time. Adventure Time is a potential genre-defining show. It represents a step away from the industry dogma that cartoons are, first and foremost, a children's product that should be exploited as much as conceivably possible. It's a true creative enterprise derived from the singular vision of its creator and nothing else. It doesn't suffer from the need to be random (like some cheap Ren & Stimpy/Spongebob copycat). Rather, it appears to espouse randomness in a world that is, in fact, designed in a consistent way with specific rules and a central theme. It has fewer boundaries than MLP and its humor is much more variably sourced.
Additionally, while the six main ponies are all pretty strong characters, Finn and Jake have the benefit of exclusivity that allows them to inter-develop along a much quicker path. Their bromance has evolved more rapidly and, as a result, they have a much more unique and interesting chemistry compared to the MLP gang.
tl;dr - Both shows are great. MLP annihilated my expectations, but Adventure Time may be the best cartoon on television.
