I've run the old STT, the ST-Maxx, the S/T, and now have the STT-Pro (that I won here on Expo; thank you, Expo/Cooper).
The STT-Pro works the best in GA crappy clay. The ST-Maxx works better on our roads ----- but the difference isn't dramatic like the difference between a street tire and a hard mud tire.
Both the Pro and Maxx BLOW AWAY the older tires with the older rubber compound both on and off road.
Spending my money, next time it's the Maxx for me. If I could get the Pro for the same money though, I'd go Pro. The Pro is pretty sharp looking...
I just got back from about a 5,000 mile roadtrip, btw. Only about 200 miles of it offroad, a mixture of dirt and beach. The Pro was very composed on the highway, backroads, anywhere. Absolutely no complaints.