Mauser for me
The Mauser action had been made in dozens of cartridges, millions of sporter rifles are patterened on, if not made directly from the Mauser action. I have had Mauser sporters in calibers ranging from .22-250 to .458 Win Mag.
The Moisin Nagant action has been made in only one caliber, and the only sporters made on it are the ones done my individuals.
I find the russian action to be clunky and awkward.
Both rifles in military trim served well, and both are capable of adequate accuracy. Comparisons between individual rifles only tell you about the individual rifles. One cannot truthfully say one is more accurate than the other, just because the one you have shoots better than the other.
The Mauser is more refined, and vastly more versatile when you go beyond straight military issue.
One reason there is a myth about the Mauser not being good in the extreme cold of the Eastern Front is because of the oil the Germans used. Not the rifle itself. What still works ok around freezing can turn to glue at 20+ below zero. The Soviets had extensive experience in this area that the Germans did not. The Germans learned, and compensated, but it took some time, so the legend got traction.
Personally, and speaking only of rifles in issue condition, I find the Mauser to be easier to use, especially those models with a bent bolt handle.
All else being equal (and it almost never is) bigger bullets tend to work better.