For fuel economy, the TDI is the way to go.
How heavy is the trailer? I have no experience towing, but friends who tow tell me that a long wheelbase aids stability. Those who tow somewhat heavy trailers tend to prefer the large GM and Ford SUVs. The Dodge Durango is one in-betweener, longer than the Touareg but not nearly as bulky as the big ones. The related Jeep Grand Cherokee is offered with a diesel, but it's also similar in length to the Touareg.