If you're a beginner, performance and portability should be a secondary concern: Create a game worth playing first.
This is exactly the reason I am trying to switch from XNA to another language. I think that for a beginner developer, smart phones are a good platform to get some experience, and get real users to test and evaluate your game. With that in mind, I think that XNA (or WinPhone's to be precise) has way too few users - Android and iOS are much more represented. I know that Android uses OpenGL (and Java, I think), it seemed as a good language to make the transition to (iOS and iPhone's are a bit too expensive). Can you please comment on this logic?