We currently have a 300 win mag in a Vanguard Accuguard, and a 300 PRC in a Bergara premier. I bought the Vanguard a very long time ago, and when I did, a gunsmith friend suggested I re-think it because if I ever wanted to make significant changes, it would be better to have a Remington pattern action. I ignored him, thinking I'd never make many changes, to my detriment. The box works fine for Sammi spec ammo, but if you want to shoot the newer high BC bullets that make it really hum, you will need a lot more space. So now years later, I had to have my action opened up and a new box put in and that cost me. Trigger, I put a Timney, but I only had the choice of that one lower end Timney, not the higher end ones, and not the one I want. Manners, McMillan and a few others make some stocks (mine came with a B&C), but the choices are not there, especially in chassis. So my opinion, long and short, whatever you get, get Remington pattern.
That's why when it was time to upgrade my son's gun, we went with a Bergara. Doesn't have to be them, but I'd still recommend something that takes Rem stocks, triggers etc....and make sure the box is 3.7 internal or longer still.
In terms of PRC or win mag...I guess it depends on what you want to shoot. I shoot 212's and up. For win mag, that's a handloader's option only. PRC, it's off the shelf. On the other hand, there are few choices in PRC, but I expect that will change eventually. Basically, most would agree you can do most things with a win mag you can do with a PRC if you handload and have a long box. The exception being the real heavies....then the faster standard twist of the PRC's and custom win mag barrels come into play. But keep in mind, with that faster twist, you definitely will notice it in your spin drift vs a 10 twist barrel.
One concern on the PRC I should mention, there is a thread on a couple forums, but particularly on Long Range Only, where some PRC owners are noticing that due to the dimension designs of the case, after a few loadings, some case brands cannot be resized back to fit in the chamber....or worse, it sticks in the chamber after firing and has to be pushed out with a rod...and small base dies will not fix it. It has to do with the dimensions of the case, the max size of dies, and the proximity to the web in the case where the problems is. Apparently, it is worse with tight chambers, and it may also be worse with the better brass, ironically. To test, I took 1 piece of brass and loaded and fired it 8 times and we had no problems, so my son's chamber must be a little looser in the area just in front of the web. I'm no expert on this, but folks are reporting this on both PRC's (300 and 6.5), you can read up on it at LRO. I talked to Forester about it, they heard of the issue, but have not seen it on their end at this time.
Anyway, it's not been an issue for us, so if I were you, I'd pick a rifle that accepts Rem aftermarket parts and make sure the box is long enough. If you are a handloader, pick either....probably does not matter, you can ignore the belt on the win mag by just bumping the shoulder and it will headspace off the shoulder and give you the same case life. But if you want to shoot 212's and longer and don't handload, has to be PRC. If you want to shoot 230's, due to standard twists, PRC.
HTH....