[MeSparks]

I'm planning to upgrade a Spec Roubaix triple to Ultegra derailleurs front/back.

Should I be using a long or short cage rear? Why would I select one versus the other?

Thanks

 

[MeSparks]

If you're staying with the triple crankset and just changing f/r der. you'd stay with the OEM (long cage) RD.

FWIW, unless there's a reason to take the opportunity to upgrade, IMO it's not necessary, because you'll see no difference in performance with this change.

Short or long cage RD's are chosen based on total tooth capacity of the drivetrain.
Example: 5700-GS RD total capacity = 39T, 5700-SS RD = 33T

Its more about the positive locking adjustment screws than performance.