If you will be mounting the bellows to a tripod, or a big stand that does not have nice fine-focus ability, you need the focus rail, and in this case you have just a few choices:
- Nikon PB4 or PB6 (Nikon F mount)
- Vivitar Bellows System (T-Mount)
- Canon Auto Bellows (FL or FD mount)
- Novoflex (various mounts and combinations available)
- Pentax Auto Bellows (M42 mount)
You could also buy a separate focus rail and use the following:
- Vivitar Triple Track (M42 or T-Mount)
- Spiratone RapidRail (M42 or T-mount)
The PB6, Canon FD, and Pentax all have the similar precision nylon bearing and all are good choices from adjustability perspective.
The PB4, Novoflex, Spiratone and Vivitar System have round rails with metal-metal interfaces and are acceptable precision but often are more difficult to adjust.
The Vivitar Triple track has a center rail for the adjustment and locks on both sides, so eliminates the binding problem and locks down very rigidly. But these don't have integrated focus rails!
Height above the mount varies widely between bellows. The Vivitar Triple and Spiratone have very low heights, about the same as the camera itself, so are suitable for certain apps (like mounting on a microscope stand). This also reduces the tendency to vibrate a bit. All the others, with the integrated rails, sit much higher, with the Pentax having the lowest standoff and the PB4 and PB6 the highest.
Ultimately, my preference for bellows with integrated rails is the Pentax, as most cameras will need adapting anyway, and the M42 mount is easy to find adapters for either end of the bellows. Second choice is Vivitar System, as T-mount has same considerations as M42. Third is the PB6, but only if you are shooting Nikon.