[QUOTE=Riles]
is one better for shifting that the other or is simply preference ?
QUOTE]
I've been running both for almost a year now, with approximately equal usage, and can honestly say that I don't think that there's really any difference in shifting performance, so for me it would just be preference. There is some kind of shimano press release type thing that I've seen that says which one is better for what, but I can't remember what it said.
Capacities of deraillers is on the shimano website to compare with your cassette to decide long/short cage.