I think I’ll finall give CX a shot this year. I’ve got a frame built up w/ old MTB parts and a flat bar, but have an old drop bar to swap in. I plan on going 1 ring up front: 39T. I’ve got a 11-32 XT cassette, so good gear range. I have the XT rear der on now, but have an old 9-speed ultegra to use as well. Will it make any performance difference in the wet and mud?
Not sure I know enought to argue performance but I run Ultegra 9 in back and always wish I had more gears. I haven’t had any problems shifting in either mud/snow/sand whatever
if you’re running 1 ring in the front you’re going to want to use the full range of that 11-32 cass. An XT der. will shift better through that wide a spread than an Ultegra der. will.
Shimano states that the 9-speed Ultegra is designed for a maximum of 27 teeth on the rear cassette, so I’m not sure if it will work well or not… I seem to recall when my kids were younger I put 11-32’s on their road bikes for courses with really steep climbs. Did it work? I’m pretty sure. Did it work well? Hard to trust feedback from a six or seven year old 
Regardless, I’d guess that it would not shift any better than an XT which IS designed for 11-32/34.
I would definitely go with the XT (or LX) rd over the Ultegra. The ultegra is not designed for that high of a gear range - it will work if ideal conditions, though it may take some ‘coaxing’, but would be less than ideal once things got messy. You really want the longer cage and the stronger return spring of the XT. Also, I would go with long cage over medium if you have a choice.
Long cage with a road double? Why? You could probably get away with a short cage and have less chain and quicker shifting. He just needs to choose his chainrings and cassette and then calculate his chain wrap and get the appropriate cage length.
Probably should add this: To calculate chain wrap capacity determine the difference between your largest and smallest chain ring (e.g. 48-39= 9). Then determine the difference between your largest and smallest cog (e.g. 32-11= 21). Add the differences and you have chain wrap capacity (30).
I guarantee that with your eyes closed, you would not be able to tell the difference in shifting performance between long cage and short cage RDs in the same double drivetrain. Sure, the shorter chain and shorter cage weigh a few grams less, but the shifting speed is so good for both these days, that performance is not discernible.
you’ll need to use the XT to hit hte 32t cog.
Thanks all, the XT it will be. Its a long cage. I’m pretty sure it will, but will the XT rear work w/ the ultegra STI shifter? Any adapters needed?
XT should work just fine with your Ultegra STI. Now if you were running Campy you’d need a little pulley thing by your rear dérailleur - I think it’s called a “travel agent”. But you ought to be just fine without anything special.
Will work fine because the indexing is in the shifter, not the derailleur.