I'd also suggest the move to a Medium Length action. It will really opens up the possibility of running longer bullets. I have light weight mountain rifle short magnums in 6.5 to .338, 2 are XM length and 2 are short action. I don't notice a difference in the handling of either rifle, but certainly notice at the reloading bench. The short actions can still be loaded to 2.990" and it still feels too short. If I had another chance they would all be XM length and I'd knock an inch off the barrel to maintain a shory overall length.
Staying without a Muzzle brake, I'd personally limit myself to the 7saum. My 300wsm in a 6lb 2oz rifle without a muzzle brake has too much recoil for me and it does affect accuracy. If you can handle the recoil, then I'd take 300wsm over the 7saum and would probably consider a muzzle brake for load development and practice vs using a lead sled.
If you are willing to go wildcat, I do find the Sherman Shorts to have more manageable recoil then WSM. They also handle longer bullets in Short Actions better then a wsm.