I'll answer you calmly. None of the three are interchangeable. The physical difference is in case shape and design. All will shoot the same bullet. In ascending order of average performance (read velocity) you have the .300 Win. Mag, then the .300 Weatherby Mag, and the fastest of the three the .300 Remington Ultra Mag. ON AVERAGE there is about 150 fps separating the three.

I would have to say the .300 Win. Mag is most popular with most availability of ammo and probably a toss up between the Weatherby and RUM.