I like Mario a lot, and I use libgdx for some of my stuff (and am looking at contributing some gamepad stuff to it when I get around to writing it), but despite its quality--which is very good, and the primary developers are professionals who know what they're doing--it is not in the same conversation as MonoGame.

No Metro support (and I find it very unlikely Microsoft will allow Java apps in their Metro store, and almost definitely not on ARM devices) and a reliance on IKVM on one of the major mobile platforms makes it a poor choice for greenfield development if you care about those platforms.

It's really, really great for desktop and Android, though.