Or you could just stick with HTML and go for PhoneGap, gives you just about every mobile platform you could ever wish for.
For your data exchange, JSON is probably the best one. It’s much less verbose than XML in just about any case. In any case, for mobile devices (and even for a snappy desktop experience), you want to build your app to transfer the least amount of data possible. Plenty of resources about that around the web.
Peter De Berdt