My understanding of PhoneGap is that they provide Javascript APIs to much of the iPhone APIs.
Titanium seems easier for a web developer background. It is a simple XML file to create a basic TabView application and then everything in the content area is controlled by HTML / JS. I also know that Titanium does provide some javascript access to some of the frameworks (particularly access to location information, the phone ID, etc).
UPDATE: Titanium added Maps API in version 0.8 of their framework.