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I vote S - why pay a premium for the GTS if it's for towing. The GTS handles better than an S, but you're not looking for that. You would get lower gearing in the GTS if that's important to your towing needs.
I have had an S and a GTS. Both were used as my daily driver and my tow vehicle. I love/d them both and both have preformed faultlessly. The only change I have made is that I bought a set of 18" wheels for the GTS when I am towing. A set of tires for the 18s can be bought for the price of one 21 inch tire and The soft, high performance 21 inch tires tend to wear out very quickly when towing. You won't regret your choice either way but if the car is primarily fit towing then I also would think the S offers better value.
Chaos, any possible interest in an '06 Cayenne Turbo S w/31k? I'm more than likely going to put mine up on the market in the next few weeks due to 3rd child being born. I love the PIG but traveling any distance with 3 kids (2 in car seats) is a joke.