There is also the indifocus shoulder mount and cavision that are around the same price as the Redrock shoulder mount. They all probably work well. The cavision seems to have the most accessories for it, but you probably can use the others with what accessories they have and not miss out. Zacuto is probably going to be the most expensive but they seem to have definitely the most accessories for any setup.
There are also other shoulder mount systems that aren't based on rails like the DvTev Multirig and the Varizoom DV Media Rig though these too run a little more.
I've used a rail system shoulder mount and a Varizoom DV media Rig and preferred the varizoom for handling but rail system shoulder mount worked better for going back and forth from a tripod to shoulder.
