I have Mk 25 EVO with G260 regs and Mk 25 EVO with S620Ti regs. I also took the SP class on reg servicing a few months ago, which included the A700. I have not breathed off the A700.
To me, the G260 breathes just a little nicer than the S620Ti. The 620 may have better WOB numbers on an ANSTI machine, but I believe that does not tell the whole story. I am buying a couple more regs shortly and it will be 2 more G260s. The 620 is lighter and less bulky and the ones I have will be dedicated to single tank diving and travel. My regs with G260s will be used for technical diving (sidemount and backmount doubles sets). I also have Dive Rite XT and Hollis DC3/212 regs. To ME, the 620s don't really breathe any differently/better than those other regs. They all seem about the same, to me.
I loaned a reg set to a friend last week. She used them for 13 dives while we were down in NC. It was a Mk 25 EVO with 1 G260 and 1 S620Ti. She compared them both and said that she could tell a difference and the G260 breathed a little nicer, in her opinion, as well.
I was pretty hot to get A700s until I took the SP class. I will not be buying any for myself simply because (as
@rsingler already alluded to), the air barrel is brazed into place and it's not hard to ruin the whole reg body by accidentally trying to turn the air barrel with a wrench. Plus, replacement of the body (whether it was ruined as described or just because somebody dropped a tank on it or whatever) is $200 vs $20 for a replacement G260 body. And there's the whole thing of needing a tool to remove the face plate on the A700 and having to make sure you don't drop/lose any of the screws. I would not want to be trying to take the face plate off of an A700 while on a pitching boat. Say, because it got some sand or sediment in there that wouldn't come out on its own. OTOH, if I had to, I would feel fairly comfortable doing it underwater, during a dive, if I NEEDED to take the face plate off a G260.
Bottom line: The G260 is an awesome breathing reg and, bonus(!), it's the least expensive of their "top line" regs.