I know I'm a bit late to the party, and the decision I think was already made to the 6.5-06.
The 6.5 PRC was generally built on short action. Can use long as well, to get more length for your bullets. That said, Defiance has their XM action. A mid sized hybrid sort of that will work as well. The stock makers are finally catching up to these and are now offering stocks in the XM length.
I had one built on the short action and had to have the ramp lengthened by my smith to clear the 6.5 PRC. Some of the box mag manufacturers are also now catching up and offering a hybrid mag length as well, but they seem to be the last to the party.
I am using the 156 berger EOL and get no problems in groupings, with consistent sub .5 moa and better, and near max mag length in an MDT mag for the wsm ammo offering. Waiting on the new Hawkins offering on the flush mag for my chassis to be available in Canada and will step up to it to get a little bit more mag length.
I'm not a speed chaser, and load for groupings, and yea I know that is beyond some. My strelok puts me into around 2930 fps for speed with the 156 gr bullet sitting above H1000.
So you are getting more options out there being available to build on a medium action.