Out of the two options you listed... i personally would vote for Windrock. I have put over 1500 miles up at windrock and still have not been on all of the trails. They own over 75,000 acres..and are constantly putting in new trails. As people have said, you can go from mild to OMG in a matter of .2 seconds. They trails are also constantly changing up there. You might go on trail 16 for one run... come back 2-3 weeks later and trail 16 has changed completely on difficulty. It all depends on our weather, and amount of use on the trails. The trails can get hogged out pretty bad..and quickly too. If you do go to Windrock, please buy a map. A lot of people wheel the southern side of the map and don't go too often to the northern side... the north side is where some good LONG trails are at. Putting it this way.. we averaged about 6-8mph in SxS's and it took us from 7am roll out... to back on the trailer at 2am. That was very few stops and we had to come down the mountain once for fuel for the 2 4wheelers that were with us. That one ride was about 150 miles... 90% trail.
If you have any questions about windrock, just let me know!
-Thomas