Apps build using Titanium are native and they can’t be reused among different platforms. Code once, deploy everywhere. PhoneGap promotes code reusability and portability so that same code can be deployed among different various mobiles, tablets and TV with minimal change.
Phonegap is MIT licensed and Titanium is Apache licensed. How that help in selecting a framework from technical perspective.
If you are looking to create an app with native look and feel using your existing web development skills Titanium is the right choice, but If you want to be able to “port” your app to various platforms and devices more easily, PhoneGap is the right choice. There are more than 80,000 apps developed using PhoneGap as of July 2012.