The differences are mainly in the way they feel & fit you, can (or can't) be adjusted to do so out of the box, and whether you can hang extra things off of them or customize them from stock. My brother's preferred approach was to take the American Predator and replace the stock. That got him some or much of what the RPR stock offers for fit. Then he put a Timney trigger in it. That got him a better trigger than either gun comes with. He's happy, but he spent just as much as if he'd bought the RPR in the first place, or maybe a skosh more (I don't recall the exact tally).
There's the possibility that the RPR's alignment of stock vs. bolt & barrel does indeed slightly reduce barrel rise from recoil, but that's something I haven't tried or been able to test.
