The advantage of the RUM cases is with heavy bullets, long barrels, and slow burning powders. They can be hard to get them to shoot with the light bullets. At least this is what I've heard.
There was a lot written on here a few years back about loading the 300 WSM
with the 125 BT. Supposedly getting near 4,000 fps. But I believe most thought that could only be done at unsafe pressures. But If you need to do this at 7mm or 308, the WSM case would be better than RUM, in my opinion.
For my taste a 243 loaded with a 55-87 grain bullet would be nearly as fast, or faster, and cheaper to shoot. The 243 shooting a 55 grain BT at 4,000 fps is a blast. The BC of that little bullet isn't up to the 125 BT, but these are really only medium range loads anyhow. Out to 300-400 yds the BC doesn't make much difference.