You really should consider 6.5x55!
Ruger chambers the M77 in 6.5x55.
http://www.ruger.com/Firearms/FAProdSpecsView?model=7895
As does Sako, Tikka, CZ, Remingon, Steyr, Cooper, Blaser, Winchester, Baikal etc.
6.5x55 performs better and doesn't have any real disadvantages. They are not as closely matched as one might think.
I'll go ahead and try to make the argument for 6.5x55 over 260, though I'll admit it isn't superior to 260 or 25-06 but has some advantages.
Ammo availability- Adavantage 6.5x55
There are actually more commercial loadings available for the 6.5x55 than the 260 Remington. PMC, Remington, Winchester, Federal, Norma, Hotshot/WolfGold, Hornady, etc. etc.
Number of factory chamberings- Advantage 6.5x55
I am not entirely sure about this one, but can think of more companies chambering 6.5x55 than 260 Remington. Anybody know about this?
Medium/Large game peformance- Advantage 6.5x55
6.5x55 handles heavier bullets better, and usually has a twist rate optimized for these bullets. 260 Remington is intended for use with 120 gr bullets. I'll explain this better later.
Varmint performance Advantage 260
I am basing this solely of the usual twist rate. Neither cartridge is really an ideal varmint cartridge.
Handloading potential- Advantage 6.5x55
More flexibility with OAL, better components available, etc.
Case Capacity- Advantage 6.5x55
6.5x55 ~53 gr of water
260 Rem. ~50 gr of water
First off 6.5x55 is underloaded in deference to older rifles. It is up to modern standards and can be loaded to modern pressures when appropriate (like in a Ruger). Simply comparing published data and factory loads does not give you a good indication of its peak performance. Though it is interesting to note that even downloaded it is still wonderfully effective, and all of its reputation as a superb medium game cartridge were earned at these moderate velocities.
260 max OAL = 2.8"
6.5x55 max OAL= 3.149"
6.5x55 is a medium length cartridge. This makes me curious as to what Ruger did. I would wager that they are actually built on a 30-06 length action. This is a good thing for the handloader because in theory you have plenty of length to work with. You could load to 30-06 lengths. My handloads for my M96 are approaching 30-06 OAL. This is especially important when loading long heavy bullets which are what makes the 6.5 so interesting. A 6.5mm 140 gr bullet is rather long (a good thing). This gives excellent ballistic properties. However, because 260 is cramped for OAL this means that a 140 gr bullet extends into the case. Not good. 6.5x55 does not have this problem. THe interior of the case is for holding powder, not bullets.
Here's a big advantage IMO. If you handload, you can get Lapua brass for the 6.5x55 which is as good as it gets. Remington brass is kinda crappy.
Really you would think that
More rifles are available in 260 Remington- Wrong
More factory loadings are available in 260 Remington- Wrong
260 Remington peforms better- Wrong
The only reason I can think of for purchasing a 260 over a 6.5x55 is if you want a short action over a long action (which is also full of exagerations and misconceptions).
A 6.5x55 has better performance (especially with heavy bullets) than 260 Remington and gives up nothing in availability of ammo, rifles, components, reloading dies.
I really can't think of a reason to buy a 260.