With 180gr handloads the 300win mag will produce 3100fps and deliver less recoil while doing it.For the sake of 50fps,I would stick with the 300win mag.
Why would you do that? The guy already has the gun, he found a load very quickly and is quite happy. Does your animal know the difference? Should mossy go out and sell the gun right away cause it doesn't meet your criteria of 3400fps? I mean the guy has a gun that he is happy with, and it would be stupid to take a loss on a trade just to try to get to a "hype or bragging" number. Seems to me anyway.
And you are forgetting an important thing. His 300RUM has the option to go faster and with more powder. Your 300win does not.