buying rear mech for the Compo, to be used (for now) with an 8 speed cassette. Might run a single chainring upfront (although as yet undecided) which if so will be 38T. Should I get the short cage version?

if you are just going to run a single ring then there is no reason not too. You could also run a short cage roadie mech if you wanted too.

if it's triple ring setup then you'll be wanting a long cage mech really.

My fave choice would be a mid-cage XT from the mid to late 90's. Built likt the proverbial brick outhouse.

If your going to run a single ring up front, then a short cage rear mech is a good idea. i run a Shimano sora roadie rear mech, cost around £15, works well and is cheap to replace is u smash it, only weightier and a uglier than the new XT version. but hey.. who cares bout that!

I've currently got a mid cage XT on my Cotic, but am going to get an Ultegra next. All the new Shimano MTB mechs looks fat and ugly these days. Even with triple rings, so long as you stay out of the small sprockets in the granny ring, a shorty will be fine. Shorties look cooler too :-)