Is there any difference in factory ammo availability between the 6.5x50 vs the 7.7x58? That may determine what I look at first.
7.7 is a little easier to get ahold of but Cabelas has 6.5 so it's not too bad. grafs has store brand stuff in both calibers loaded by hornady and uses PPU brass but so far every time I've looked it up they are out. PPU used to have loaded in both calibers but now they just make brass for hornady and a couple dinky brands.