OK guys, I am just starting to load for a 260rem. This will have a shortish - 21" - barrel and a sound moderator.

Plan on loading 120gr Sierra's. I have both Benchmark and H4831 to hand, funnily enough these are on the opposite extreme ends of the burn rates for this cartridge.

As per Hodgdon's max load with benchmark is 40gr's, max with H4831 is 50gr's. Theoretical velocities are with both in the 2900 area.

Is there any advantage in using as fast a powder as possible over the slowest, or vice versa, in any given cartiridge? Other than the obvious reduced usage of the fast powder that is.

I think I am really asking do you go fast, slow or somewhere in the middle for best results?

The slower powder should give slightly lower pressurtes for the same velocity, but not always. The muzzle blast will be louder with the slower powder too.
That said my two 260s shoot best with powders in the R19/H4831 range and I`d personlly try to stay with something no faster then Varget or 4064 with any bullet weight.
JMHO

i've used ww 760 and h4831 both with the 120's. had good success with either. actually i don't think the 260 is at all particular which powder is which

H4831sc is a great choice for highest vel loads w/ 120gr bullets.

RL-15 has worked the best for me with outstanding accuracy with both 100gr Nosler BT and 120gr NP.

I have tried 4831 4064 4895 H-380 4350.