My vote is for the 264 and 140 grain bullets. Just got mine back from the gunsmith and will try it on antelope and deer this fall with any luck drawing a tag. I sold my 270 wsm a few months ago. I shot a 270 win for years and won the 270 wsm in a raffle - it just never quite did it for me. So, now I'm on to three main calibers a 25-06, a 264 win mag, and a 300 wsm for now....
It seems to me that when I was kicking back and forth on caliber about a year ago, the ballistics that I looked at suggested that the 264 would beat a 270 WBY beyond 750'ish yards. I'd have to look back through some of my research though.
Either way, dead is dead and a good shot with a flat shooting .264" bullet vs a flat shooting .277" bullet shouldn't make a huge difference.