Previous discussion on Stackoverflow:
Comparison between Corona, Phonegap, Titanium
A good clear explanation from Jonathan Stark:
http://jonathanstark.com/blog/2010/04/05/phonegap-vs-titanium-mobile-et-al/
In terms of your questions:
To check which native features are available for each framework you will need to check the API docs of each of the frameworks:
The phonegap ones are here http://docs.phonegap.com/
The advantages/disadvantages I think are covered in the two links above.
It will depend what you are doing but I imagine you will have some differences in your code for each platform.
Hope that helps!
