First lets kill one myth right now...brass prices. 300 wby brass is CHEAPER than RUM brass. Sinclair sells Rem brass in both. Cost: 300 wby...74.55/100. 300 RUM.....92.45/100. Now if you plan on beating the crap out of your brass, then use the remmy brass in either and throw them away after a couple of firings. I've been there and done that. Max velocity is ALWAYS fun. If you want to keep things at realistic velocitys, buy GOOD brass. 300 WBY Norma 134.85/100. WAY better brass than remmy brass. 300 RUM, You may be stuck for a while....I know Nostler is making them, but, availabillity is poor as of now. I have no experiance with that brand of brass. I have herd that is not as good as the norma/lapua brass. As far as cost. It looks like the other calibers that Nostler is making is comparible in price to Norma.
Accuracy.....Belt.....Sholder design. ALL HOGWASH. Check the 1K results. Hardly anyone out there using a 300 RUM. There are quite a few 300 wby's, and a bunch more 300 improved (AI, BAER, Spencer,.ect...). And they win!!! My point is that the belt has nothing to do with anything once your brass has been fired, assuming you know what you are doing at the reloading bench. Even at that, I have won matches using new brass. And, that being said, and the inexperienced loader over sizes his RUM brass and pushes the sholder back to far, you have headspace problems. Push the sholder back too far on a belted case and....yep...you still have the belt to headspace off of, keeping things safe. I will admit that one of the reasons the 300 RUM isn't prominant in 1 K benchrest is the lack of QUALITY brass. I think it can be just as accurate as the wby/AI/ sized cases. BUT....Compeditors are now leaning to the 300 WSM. They are finding that they just don't need that much case capacity for thier use....but we are talking LR HUNTING here, so lots of boiler room is good. But as for the 300 RUM....mayby to much of a good thing.
Now, based on capacity alone...all else being equal, the 300 RUM will be a touch faster. As far as free bore, I know that WBY factory rifles have some, but I admit I don't know about the other gun manufacturers out there. I shoot all custom stuff in 300 WBY, and don't use ANY free bore. A little less speed....a whole lot more accurate. But, watch for high PSI...it will come SOONER w/o freebore.
Good luck, and know this.....If your biggest wory in life is haveing to choose between these two choices, then life IS GOOD!!! No bad choice.