Would it be possible to use RubyMotion and PhoneGap together? It seems like you could start your app in PhoneGap, then migrate towards RubyMotion as needed.
If you are going to eventually use RubyMotion, why not just start with that?
I have been using it for about a week based on a recommendation from another HN'er and I have been extremely pleased. It is all new development, though. I am not especially good at Cocoa or Ruby, but have found it a very enjoyable experience.