I've never fired or even seen a 300 RUM being fired so I'm no help with that.
The 300 Win mag is more familiar to me.
Currently from what I know of available components is that only Remingtom is making the RUM brass. I've heard of QC problems with the Rem brass but again have no experience with the 300 RUM.
The 300 Win mag has been around a long while and nearly all the major manufacturers make brass, there's some very high quality brass for it.
If I were to build a rifle for strictly long range shooting/hunting and wanted to push the big 220 and 240 Sierras I'd probably go for the 300 RUM and a extra long barrel ("a Darryl barrel"). But if I were going for a hump around type hunting/shooting gun with a stubbly little 26-28 inch barrel
Just noticed your location in Montana. There is a 1000yd BR range right there if your not aware. Myself or Darryl C. can get you in touch with some of the boys at that club. They have a 11# class that they shoot there along with the 16.5# and heavy Gun class also. It would give you some good experience and you would have a bunch of fellow shooters to ask all the questions you wanted and get to see and touch the equipment first hand.
Just a suggestion only..... Good luck with your new toy. A 300 Win Mag isn't the newest wildcat on the block, but the ol' caliber still gets the job done in the accuracy and killing departments both. Have fun.