I have an Acratech GV2 on the way which is quite similar to the Acratech GP. I opted to not get an RRS head in favor for Acratech's "open" ball head design. However, I am pairing the Acratech GV2 with an RRS Lever-Release Clamp because I enjoy the convenience.
After doing some research and considering what other experienced photographers have said, I chose the Acratech because I feel the "open" design of the ball head will be an asset, especially for landscape photography. It will be easy to clean if anything gunk or grit gets on the ball itself. For a head like RRS, if fine grains of sand got to the ball because of wind, lets say on a beach, you would be hard pressed to clean it yourself and will more likely than not have to send it to RRS.
Also, the Acratech GP (1 lb) is roughly half the amount of weight as the RRS BH-55 (1.9 lb). If weight is a factor, I'd strongly consider the Acratech GP.
As someone mentioned earlier, the Acratech definitely has more of a "cool factor"
Both heads are great though, so good luck!