wilkeshunter,
Here is my take on the comparison.
QuickLoad lists the following case capacities (grains of H2O)
.260 Rem 53.5
.257 Rob 56
6.5x55 57
If all else is equal - barrel length, muzzle velocity, bullet weight (e.g., 100 or 120 grains), bullet type (design), chamber pressure, specific powder, etc. - you're left with bore diameter (very close) and amount of powder.
The .260 Rem would have a very slight advantage over the other two because of the smaller case - it should, theoretically, require less powder to reach a given chamber pressure and muzzle velocity for a given bullet and powder. However, I think the difference probably would be very difficult to discern.
One thing to note is that the Speer #13 manual indicates .260 Rem start loads using their data shouldn't be as much below max loads as is typical for that cartridge capacity. Speer #13 .260 Rem starting loads are 2 grains below their max loads (their start loads are about 5% rather than 10% below max) with the explanation that pressures drop rather fast in the .260 Rem as powder charge weight is reduced. That would be something to consider if wanting to load reduced recoil loads, but I don't know why Speer identified that as an issue with the .260 Rem but not the 7mm-08 or .243, which have very similar case capacities (55 and 54 grains H2O, respectively vs. 53.5 for .260 Rem according to QuickLoad). That potential issue wouldn't be an issue if you were planning to load at least moderate to full pressure loads.
