I have the smaller kirk ball head (BH-3) and have been very pleased with it. I think it is rated to hold 15 pounds and it has full size knobs, good for winter in MN
I had also looked at the really right stuff ball heads. They had one similarly sized and one that was even smaller/lighter, with about half the load rating. Your 5D2 + 135L would be under 4lbs so I'm sure it would be strong enough.