I had a Ref3 with a few hundred hours of burn-in on loan for a few days last year, a week after having the LS25mk2 on loan. My preamp at the time was the SP16L, but as stated above I now own the LS25mk2. Source was an ARC DAC-5, amp was an ARC VS110, speakers Totem Hawks.
The Ref3 just seemed an order of magnitude more detailed and precise, but the LS25mk2 had more warmth. I've heard similar comments be made about the LS25mk1 vs mk2 - that the mk1 is warmer and some people prefer it that way.
One feeling I got from the Ref3 was that of a mismatch with the rest of my gear. It's really tough to describe this, but the 25/2 just seemed to fit in much better with the signature of the rest of my chain. I would have given the Ref3 more serious consideration if I had the budget to drop another 20k into upgrades soon after, but that's a path I just didn't want to go down.
My advice to other LS25mk2 owners would be to consider the balance in your system and how the LS26 might change that. Go and listen to it, but be prepared to shift the rest of your system around a different sonic signature if that's the direction you want to move in.
This would be a much easier decision for someone wanting to move into ARC's midrange systems as they would probably already have multiple upgrade steps planned. The rumored replacement to the VT100 would probably be the best paired amp - I don't believe that the VS110 or VM220 would feel at home with the "new sound" given my experience.
For me, I will probably revisit the Ref3 in a year or two but only when I can hear it with a pair of Reference 210 power amps. I can't see moving sideways to the LS26, but I might be convinced to move the whole thing up to that next level. The problem there is that ARC doesn't seem to have anything in that next range - the Ref3/210 combo costs a whopping $30k! It's at that point that other alternatives will probably get auditioned first.
I hope that helps.