My .02
What no one has seemed to mention is the ammo you're talking about. Both cartridges can be very accurate, as can the more common chamberings they're available in. However, I've yet to see any milsurp ammo that came close to the accuracy of good commercial and especially handloads in any of my Mosins or Mausers. The best milsurp ammo I've see was Czech "Silvertip" in 7.62x54R...Have yet to find any 8mm that would shoot better than 3" groups. But in the same gun (Scoped and bedded/restocked Mauser, but unmodified action w/ military barrel) I've shot 1 1/4" groups with commercial ammo, and right around 1" with handloads, whichisn't all that much different from an OK commercial rifle off the shelf. still playing with it trying to get it better. And A guy I meet up with, often, at the range, has an original Finnish sniper that shoots 3/4" groups all day long often better, but again this is with commercial ammo.
In short, IMHO, 8x57, 7.62x54R, 30-06, 308 (and a host of similar cartridges) all have a relatively similar accuracy potential. If you could eliminate ALL the variables, I doubt you'd find much difference in accuracy, beteen any of them.