I've had the pleasure of working with Phonegap for the past few weeks.
Interestingly enough, a few months ago I've had the chance to develop with Titanium.
It was an already existing application, so I didn't have much say in how it was structured,
but I have to say, I find PhoneGap a lot better.

The main advantage that PhoneGap has on Titanium is the fact that you're
basically coding a web app, only it's deployable on multiple mobile devices.

Whereas with Titanium, the odd JS Api that you use is kind of a mixture between web app
android SDK coding, which gives you the worst of both worlds if you ask me.

If you have an existing Web App, you can deploy your app using PhoneGap
on most mobile devices In a matter of minutes.

Basically what PhoneGap gives you is a browser within the phone, but it also provides you
APIs to the device's functionality such as Camera,  Contacts, Storage, Geolocation, etc.

It's also really interesting using new capabilities of HTML5 on the app such as Local Storage,
Some crazy kooks have even implemented a SQLite plugin !

So to summarize it,

Titanium Okay

Phonegap Awesome!