Kirk brackets are easier for me to get in UK as RRS only deal direct. I've tried both however all my brackets and lens feet are from RRS despite the hassle.
There isn't a lot of difference really. The RRS items just feel slightly and only very slighty better. One thing that does mark the RRS apart from Kirk is the centre line markers. On Kirk products the centre of each axis is marked by a notch on the bracket body, on RRS it's normally an etched marking. It's a miniscule little detail but I just prefer the RRS etched marking to the Kirk notch. But to be brutally honest, it's very difficult to choose between them and they both do an admirable job.