So, from what I can understand, each page gets its own value of the URL for the prop and evar. If that is the case, the instances should match.
About the numbers differing for the prop and evar, I really think this is cause of allocation.
Since this is a page URL that you are capturing. I would say use the prop. If you are firing the prop and evar on a s.tl() call as well, then I do not see why a prop would not work for all your needs.
Is there specifically something that I am not thinking about....
-Hyder