There's quite a bit of difference between loading for an 18.5 inch vs a 26 inch barrel. That said, I'd go with a quicker powder with a short barrel, and a slower powder if one has a 24-26 in unit.
Not really. John Barsness has tested this idea and found that for a given cartridge the powders that perform best in long rifle barrels also do best in carbine length barrels.
Reloading data supplied by Hodgdon runs counter to this idea as well. I have one of their annual manuals in which there is data for several cartridges in the rifle section with typical barrel lengths and the pistol section with abbreviated 15" barrels. These include the 243, 7mm08, 308 and 30-06. The data includes a wide variety of powders. When this question came up before I looked into it and found that the same two or three powders that do best in a cartridge with a 24" barrel are the ones doing best in a 15" barrel.
