Originally Posted by ThermionicScott

I think the reasoning is that short-cage RDs are often also designed for smaller cassette cogs, so you would get a smaller chain gap with a given cassette than a "mountain" RD that is rated for larger cogs. That should shift a little better, but you can do a lot with the B-screw adjustment on the longer-cage model. And since you're considering a 26-30T big cog anyway, I don't think a long-cage RD would be so bad at shifting. (If you wanted to do a triple plus a corncob, then I'd probably try to make a "road" RD work.)