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

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2004 BMW M3 Individual
2009 BMW E70 X5 35d
2011 BMW E70 X5 35d - Totaled
2009 BMW M3 E90 - SOLD
2006 E53 X5 4.8iS - SOLD
2002 E46 M3 Convertible - SOLD
2006 E46 M3 ZCP - SOLD
2011 E70 X5 35d - SOLD
2000 E39 ///M5 - SOLD
2003 E53 X5 4.6iS - SOLD
2006 E83 X3 3.0i MSport - SOLD
2000 E46 323Ci - SOLD