Appcelerator Titanium or Phonegap for mobile development

PhoneGap vs Native Code: It depends on your requirements.
PhoneGap good for cross platform application.
if you never intended to target any platform but Android you would probably go with native Android development but if we were going to other platforms you'd select PhoneGap.
Also, it depends on what type of application you are writing.
Ex: for3D game, go Android.
A table driven data application, PhoneGap is the way to go.
Appcelerator Titanium vs Phonegap: Titanium uses only javascript while PhoneGap uses html5/css3/javascript.
but Titanium translates javascript to native code using a translator embedded in the package for each platform while PhoneGap uses WebView component to run the application which mean that the Titanium has better performance.
the last advantage for PhoneGap that it supports iOS, Android, Windows Mobile, Blackberry and Symbian while Titanium supports iOS and Android only.
So without knowing what type of app you are trying to develop and what you are good at it's hard to be specific.

The other difference between Titanium and PhoneGap is that Titanium is actual a wrapper on Native API for Objective C or Java which ultimately produce a code of Objective C or Java for android.
Where as PhoneGap works on HTML5 and always used only UIWebView for the displaying content.
As per iOS there are some limitation on UIWebView.
So its not preferable to make the Enterprise Apps on PhoneGap but it will works fine in Titanium.
Titanium supports jQuery, Dojo, ExtJS, Sencha, YUI, OpenLayers etc.
other than the Titanium SDK and ALLOW (MVC Framework) where Titanium betting a lot on ALLOW and recommending for all the upcoming development.
PhoneGap is easy to pick and start but it has got performance issues due to built in WebKit Browser inside, everytime you do deployment, you get the same Webkit browser irrespect you are on iOS/Windows or anything.

Both are very good, actually, depending on the kind of mobile application you developing, the PHONEGAP is framework where u can use html/css/js all together to build an application much alike like web based application.
On the other hand APPCELERATOR TITANIUM is pure java script(JS) based API that craeted native code.
so depending on what kind of mobile application u developing u can choice which framework is most suited from all aspects

Its also true that PhoneGap scope may not be down in future because support maximum OS(Android, iOS, Windows, Blackberry et.) and still increasing but Titanium support only to iOS and Android and Blackberry in some case. So for career point of view PhoneGap is little better then Titanium but if we talk about quality then Titanium may beat to PhoneGap in some way.