Buy once and cry once. Just get a Harris to begin with.
Rock mount is just a chinkwanese (technical term /ubbthreads/images/graemlins/grin.gif) knock-off of the harris. I tried one just to see and junked it. It just seemed cheap. After you break one of the rock mounts and buy another replacement you just spent more than you did if you would have got the harris initially. I tend to be hard on my equiptment though. Just my 2 cents.
I use a couple harris'. A 6-9, and 9-13. Both are swivel, notched leg versions. If I need to get higher I use shooting sticks. Sometimes I skip the bipod and just shoot off of a pack, and if I need to get higher, once again, I use the sticks.