With the Mosin Nagant, a lot of people prefer the pre-WWII years because the rifles are just built better. Putting an average 1930's Mosin beside a 1943 or 1944 Mosin and there's just no comparison. They were cranking them out pretty fast in the war years, with less attention to detail. Now, having said that, you also need to be aware that many Mosin available today are Soviet factory refurbs, sometimes containing parts and pieces from other Mosin rifles that were damaged and stripped for parts. This could be true of either pre-WWII or WWII era rifles. So you could buy a 1943 ex-sniper that has been paired with a post WWII stock, and some pieces from pre-WWII Mosins. Beware the idea that a 'hex' receiver means quality. Some older hex receivers were reused on later rifles going through refurbs.
When it comes to Mosins, I would question most ideas that are simply stated, such as 'sniper barrels are better', or 'hex receivers mean the rifle was built better'. You really need to do your research first, and come to an understanding of what these rifles have been through. Then take your time looking them over and picking one out.
I found this site very helpful:
http://7.62x54r.net
Btw, my own personal philosphy is that a person might want to start with barrel accuracy. An ex-sniper barrel does not guarantee accuracy. But if you find a barrel that was once good enough to be used as a sniper, AND its still in nice condition. That might be one place to start. You can always have a barrel recrowned, but its overall accuracy is inherent in the barrel itself to some degree.