I have a 300 Win Mag that I built a few years ago just as 300 PRC started to become popular. If I had to do it again, I would build a 300 PRC since it is the modern version of the Win Mag.
A few things I would recommend you do in your build with ether caliber, is use an action and stock/chassis combo that will let use the 3.805" CIP length magazines for the longest possible COAL. I would also stick to a twist rate between 1:9-1:8 (With 1:85 nominal) and plan on focusing on 208gr to 220gr Berger Hybrid and Hybrid LR bullets. You can shoot heavier bullets, but you start to push the limits of ether cartridge and will burn the barrel out fast, and 300 Norma has more ass behind it to push out the heavier bullets and handles the 230gr and up bullets better.
Barrel length will depend on what you are going to do with the rifle and if you plan to run a can on it. My self, I use my 300WM for hunting and shooting steal and planning to use a can at some point, so I with a 24" barrel. If you plan on doing both like I did, I would stick to a 24" to 26" barrel. If it is just going to be a target rifle, I would do 26" to 28" with 30" being the max.
One last note, if you do opt for the 300WM, make sure you get a barrel with a match chamber that has the deeper throat for the longer bullets.