I would get the BH-40 LR II that has the spirit level on a lever clamp. The BH-40 Pro II is the same thing but with a spirit level in a screw clamp.
When I bought my ball heads from RRS they didn't have the spirit level option. I have a BH-40 LR (lever clamp, no spirit level) on a Gitzo 1348 tripod and have been very pleased with it. The lever doesn't clear the base on a 1300 series Gitzo tripod, but you can move it to one of six positions, and mine has plenty of room to tighten and release without repositioning the lever.
I also have a BH-25 LR on a Gitzo 1128 tripod for minimum weight scenarios. But I use it very seldom compared to the 1348 & BH-40 combination.
I've used the panoramic tools from RRS, and I expect their spirit level would be very accurate, both in relation to the camera (I'd recommend a RRS L plate) and it's accuracy for actual horizon level.
RRS isn't cheap, but it is fantastically functional, absolutely reliable, and aesthetically pleasing. If it was cheap, too, I doubt anyone else would still be in the ball head - clamp - plate business.