Originally Posted by invisiblehand
You probably want to go with the XT rear derailer since it can handle a larger cassette cog sizes. The Ultegra derailer can only handle a max cog of 27t. I don't know the capacity of the Ultegra long cage derailer.
What is the axle width (OLD) of the Surly LHT? Is there an XT rear hub that can handle a 10 speed cassette?
The OP wants to use a 10 sped cassette, so right now, with Shimano, that means a max size of 27T (12-27), therefore the Ultgera is the better choice for a RD. If it's going to be a double crank then the short cage will work fine.
All the 9 speed shimano RD's will comfortably handle 10 speed cassettes, as will all their 8 speed and 9 speed hubs, both road and MTB. (Oh, except for the DA 8 speed hubs, I think)
I am using a Deore RD but I plan to change to an Ultgera GS. I use a 12-27 for touring (V nice range), with a 24/34/48 Sugino crankset (gives a great range of gears with the 12-27, and perfect spacing, but I'm experimenting with 24/34/52 this week, and I don't like it so far).
I think the OP's idea of a compact crank 34/48 and a 12-27 ten speed is great. I hardly ever use my 24, it does get a go on my local category 1 hill that has a small section of 22%, but one of these days I'm going to try to attack it just usong the 34.