From my experiences with eight 300 Win Mags (22" to 26"), a 308 Norma (21") and a 300 WBY (24"), you can only compare "apples to apples" if the barrels are kept to the same length. Otherwise numbers can easily be "fudged". Then, there are other matters, like freebore, tight chambers and barrels that can make a significant difference from rifle to rifle in either chambering.
Having said all that, I've had 2 rifles, a 300 Win (26") and a 300 WBY (24"), that gave superb results from each in velocity and accuracy. So, I've used that experience, which was considerable, to make an evaluation between the two chamberings for my own purposes. I have NO prejudices for or against either. I very much like them both.
So, here is my objective evaluation (for my purpose):
1) Given excellent barrels with tight chambers, the 24" Weatherby would make 3185 fps from 180s using either IMR 4831 or RL-22. It couldn't quite make the 3200 fps suggested by Hornady's #4. But it was superbly accurate. (But I did chronograph loads for a fellow club member who used the same IMR load in his 26" MK-V and it recorded an even 3250 fps... exactly what Whetherby advertised for their 180gr factory fodder at the time, from a 26").
2) My only 26" 300 Win was just as accurate as the Weatherby (1/2" - 3 shot groups at 100 yds), and slightly better in velocity. Top MV from that rifle was 3215 fps from 180s. (BTW, I believe BOTH should use 26" tubes. A 2" shorter barrel WILL lose 50 to 100 fps in each, and I fail to see how 2" in hunting where these are suitable is gonna make any difference in handling!
3) In my summation: I slightly prefer the Winchester for a few personal reasons, but I've concluded that apart from likes and dislikes over rifle styles, that the actual difference in performance between the two, given the above quality of barrels that are equal in length is about 100 fps in favor of the Weatherby using the same powders and bullets.
And, BTW, the 26" 300 Win gave 3000+/- fps from 200s.
And, for any doubters, consider this: Nosler's #6 shows up to 3160 fps for their 180s and up to 2972 fps from their 200s in a 24" test barrel for the 300 WINCHESTER Magnum! My velocities were from a 26", so if Nosler's manual is realistic, then the 40 to 50 fps increase I received from a barrel 2-inches longer is realistic.
And, my brass in either could easily be reused 10X plus. Never a loose primer in either!
I think (a 300 magnum in) either version could arguably be considered the best all-around rifle/cartridge combo for any big game hunting in North America.
Bob
www.bigbores.ca
