I'm pretty sure their rifles were not the deciding factor in the eventual Russian defeat of Germany on the Eastern Front.
I own both. I like them both. The Mauser does appear to be the more refined of the two. It also handles better being that it is shorter and more ergonomic IMO. You might want to consider ammo availability in your choice. Mosin is also a good bit cheaper if that matters. I would say pick a Mosin up just because they are so cheap.
"The rifle itself has no moral stature, since it has no will of its own. Naturally, it may be used by evil men for evil purposes, but there are more good men than evil, and while the latter cannot be persuaded to the path of righteousness by propaganda, they can certainly be corrected by good men with rifles." Col. Jeff Cooper, from The Art of the Rifle