Well, the Exakta mount version is more ubiquitous and harder to adapt so they *tend* to be slightly cheaper. M42 versions more expensive.

The conventional thought is that the "Pre-War" version gives nicer images; they are narrower/straighter than the post-war fatties:

http://www.mflenses.com/gallery/manual-focus/Biotar

Owning both a post-war Biotar 75/1.5, and its soviet/russian copy, the Helios 40-2 85/1.5, I prefer the latter, which you can now get a new production one for $500 on Ebay.