Edward Ingold , Mar 14, 2005; 11:13 a.m.
Both the RRS and Kirk focusing rails are intended for use on an head with an Arca-Swiss clamp. Both have an 1/4-20 threaded hole for other mounting, but may tend to twist if the head is not level from side to side. For that reason, you may find a ball head more useful in the long run than a geared head fitted with an Arca-Swiss clamp.
The clamp on the Kirk rail can be rotated 90 degrees for use with a lens plate. That's important if you use a lens with a foot, like a Nikkor 200/4 Macro or a long lens with extension rings. It's not clear whether the RRS clamp can be rotated, but RRS has adapters to accomplish the same effect (but adding height).
The clamp is fixed on the Kirk rail, and sliding on the RRS rail. This give the RRS rail nearly twice the focusing range, since the rail can slide in the tripod head mount (Arca-Swiss), while the head can be adjusted using a rack and pinion. It is also much faster to rough in the position before fine-tuning with the rack and pinion.