I have hit both and both are great. I previously played the 545 with Kuro Kage shaft and loved it. Now my nephew has it. Crushes it.
This time I went with the 765 only because it is a little smaller at 445cc (just like the new Ping G400) and I found that's big enough. I don't hit it any better with a 460cc head and the smaller 765 also swings slightly faster, not much difference. It was more that I prefer the look of a bit more compact deeper face. Nice stock shaft too.