Unless you plan on towing, the V6 is fine--especially with sport mode. I had two V6 Passats because I thought the 4s were too loud and lacked oomph. So I wanted to make sure I had a capable engine on this purchase... especially since it's more expensive than the Passats I've had and I bought this one--ending the lease syndrome.
With my Touareg, I've been over the Rockies (Canadian, too) twice, through the Blue Ridge Parkway, etc and it's done just fine. While it can growl getting on a highway [the way I do], I would say the extra 5k is either better spent on the Nav and Prem Pkg, or... 2500 gallons of gas. That's 41000 miles of driving free if you're willing to go with the 6.