Well, an answer to the original question, in my book would go to the 300 ultra. I have owned one or two of them and if you get one with a decent barrel, they shoot very well, flat, and hit hard. Additionally I have owned a 7 rum. My experience with it was that I found it to be finicky in load development. Barrel life sucks, and put another way, a STW is about as much overbore that a seven mil. can be, and still be SHOOTABLE. I have found that by the time you get it to be repeatable, you are at STW velocities. So why burn the extra powder.
The argument about belted mags and non belted mags is just like the old chevy vs ford argument. I have been shooting belted mags for 30 years, and once you learn how to set the dies, they are NO different than a non belted mag. Now I'm sure I ruffled a few feathers by that statement, but I think there are others here that will agree.
If you have a STW, then the 7rum probably isn't needed. But if you are looking for a Fast 30' that will shoot well, and aren't too finicky, the 300 rum will fit the bill.
Don't fall for that belted versus non belted speal, it's a load of crap. My 2 cents goes for the 300 RUM, leave the seven alone.