v10 vs v6tdi

Hi Folks, First let me just say how impressed I am with this forum. The knowledge and willingness to help is awesome. Heck, UBERv10 spent 30 minutes on the phone with me last night answering all kinds of Touareg questions, and he's never even met me (thanks, Chris). Anyways, here's my dilemma...

noob here, but owner of 6sp manual Dodge Cummins with Edge and other upgrades for big torque >750 ftlb. It is the primary tow vehicle for a 24 ft race trailer, total weight about 8,000 pounds. The Cummins delivers 13 mpg towing this brick. Have test driven X5D and Treg TDI and BMW had nice...