Well I must say.
After spending a couple of days getting used to the UDK I can safely say it trumps Source in every way imaginable.
Lighting is much more complex and yet at the same time it is so much easier. Models (meshes) can be RESIZED!!!!! Thank god for that! The content browser is just a ton more useful over all. Hell even applying textures is easier.
I have a feeling this is the start of a long relationship with the Unreal Engine for me. So long Source. You proved an able stepping stone.