One of the nice things about Appcelerator, as opposed to Corona, is that it's open source and they have guides (iOS: http://guides.appcelerator.com/en/module_ios.html, Android: http://developer.appcelerator.com/guides/en/module_android.h...) for extending the framework with native code. So, when you hit that inevitable wall, you can drop to native code and make it work on both platforms. I find the speed of development in Appcelerator quickly outweighs the drawbacks.

That said, if you're developing a game, Corona is the better option. It's far more efficient in terms of FPS.