stay with MonoGame. they have made XNA completely opensource, and will be expanding on it, as well.
everything you've learned with XNA will carry over. EVERYTHIIIIING.
and you will be able to port to damn near any platform you can think of.
what's really awesome is that soon the guys at Mono will have even the XNA content pipeline rewritten and added to MonoGame.
Once that happens, there will literally be no need for you to have the XNA framework, you can just use MonoGame and still write in XNA code/syntax.
perhaps as a next step, you can look at importing box2d physics into MonoGame.
Official Nelson talking points breakdown: programming, mt dew, coffee, mcdonalds = 96%, guns = 3%, beer, boat, island = 1%
(4:06:05 PM) rflx: [23:03:45] <Adam1> well... he'll be running.. *puts on sunglasses* .. out of ammo. *YYEEEEEEEAAAAAAHHHH*
(4:06:08 PM) rflx: ^^ fixed