As Jackal points out there is higher cost involved with Windrock and Brimstone vs. RRB and the TWRA. I have rode all of them as of last year. Over the years I have been to WIndrock 3 times, Brimstone 1, RRB 1 and the TWRA area - New River Unit. Windrock is my group's second favorite place to ride next to Moab. We will probably return to RRB and the other TWRA areas in the future, but not returning to Brimstone.
I feel the price to ride & stay at Windrock is way better than Brimstone. Windrock has nice spacious campsites, a bathroom/shower building. Brimstone's campsites are not much bigger than a parking spot in a parking lot and has "glorified" outhouses with showers. A 4-night, 3-day ride at Brimstone cost about $25 more- per person than Windrock.
Windrock and Brimstone you buy a riding permit that is only good for those park's area. RRB you buy a TN fish & game license and can ride all of RRB and the TWRA areas- good for an entire year, not just 1 or 2 days.
As NitoxDiver pointed out, not well marked trails, absolutely NO trail maintenance, many trails closed at Brimstone. Best I could tell was they are catering more to the concert venue they built and center everything around that. There was a make-up concert the Saturday we were there and the chain of events was basically: 1000 people arrive in their RV's, trucks & trailers Saturday morning. Then all go trail riding for a couple hours, then all go out to this big hill over-look and get drunk, then off to the concert. Wake up from their hang-overs about noon Sunday, pack-up and go home. We were getting dirty looks from the hung-over concert goers sitting outside their campers Sunday morning at 8:30 AM when my group was firing up machines and headed to the trails.