Industry support.
Some of this may be a little off, but it is my general gist of it. George Gardner at GAP necked down the 300 RSAUM to 6.5 as a quick way to get the short action magnum in 6.5 with a case length. This required either necking down 300 SAUM or 7 SAUM cases, and typically neck turning. George eventually had Hornady produce 6.5 SAUM (also called 6.5 GAP 4s) brass in what amounted to yearly lots of a half-million cases (minimum order size I believe, for custom cases). GAP and a couple other suppliers were the only places to get new brass. The first run of it was soft, the 2nd and 3rd runs were better. I haven't followed availability for a while, there may be subsequent lots out there by now...
From what I recall, the 6.5 PRC was closer to what George Gardner had originally envisioned, but Hornady was swamped at the time the George wanted to do it because of Sandy Hook and other related craziness. They finally got around to messing with the development after the demand for ammo/components died down a bit.
The SAUM has a slightly wider body diameter, and is a rebated rim. Both cartridges have the same rim/head diameter but the SAUM is wider, and likewise has a little more capacity, and seems to be able to get a bit more velocity (50-100fps or so). Because the PRC doesn't have the rebated rim, it does tend to feed a little easier from magazines, but most people can get the SAUM to feed fine, too.
So it basically comes down to 50-100fps bonus vs. brass availability. Because the PRC brass and ammo is being produced by Hornady (and others??) and factory rifles are being chambered in it, odds are your long-term component availability will be better than the SAUM. 300 and 7 SAUM brass is notoriously hard to find, and the Hornady runs of 6.5 SAUM brass can be seasonal. In addition to that, a lot of people have problems with Hornady brass letting go of primer pockets (I believe this is a carbon ring/fouling issue, mostly). There is a SAAMI specification for the 6.5 PRC, and published load data, too.