X5 4.8is vd Cayenne GTS

[CleanIsFast]

I think you need to look at a few different things here. Really depends on what you are looking for.

Fuel efficiency - 4.8iS won't yield better MPG but if paying for gas isn't an issue then lets move on
Comfort - 4.8iS is very comfortable on long drives, and has enough space to carry things in the trunk or other passengers
Power - V8, similar to your GTS but not nearly as fast
Maintenance - need to be careful here. If you don't find a good example you could be spending thousands on repairs. Suspension, motor, cooling system issues most notably. Then your common window regulators and carriers.

Not sure what your budget is either. Have you looked into E70 Diesels? They are fantastic on highway drives and get great MPG. I used to own one and would get nearly 600 miles to the tank on the highway

[crystalworks]

Originally Posted by CleanIsFast

Not sure what your budget is either. Have you looked into E70 Diesels? They are fantastic on highway drives and get great MPG. I used to own one and would get nearly 600 miles to the tank on the highway

^ I second that. As an owner of both, the e70 diesel is quieter, more modern, and easily gets 25mpg highway. More can be had depending on driving style. Just last week I got ~25mpg loaded with 6 20" tires and 4 wheels from the 4.8is. The e70 is larger and carries more too. Give them a look if you are okay with diesel.