I think 450 bushmaster or 45 raptor is better for you (and I say that as a shooter and fanatic of 460 S&W) just because there is much more support for shooting 300 yards with them (bushmaster anyways) than with the 460 S&W, simply due to the nature of the guns chambered in them.
were it me, assuming I can blow open 308 pickup brass straight, I'd be going for the raptor. I don't mind expensive machinery in order to limit brass cost at this point.
Nice to go up to 65 ksi, where the bushmaster can't really go. There really isnt data for 460 S&W for those 160 grian barnes TAC bullets, but let me know if you ever find any, especially for their claimed 3000 fps. I bet I can do that in my BFR while staying under the pressure limits. The 300 grian LEE boolit would be the hammer of god though, that's for sure. If you could get it to feed.