How to quantify potential supervisor's reputation at a non-top university?
Assuming both of them are still research active, in the sense that they are publishing and going to conferences, you're looking at a lot of noise and very little signal.

Both advisors got a job at a university with a PhD program.  That already means they are pretty good and have significant research networks.  If you end up doing exceptionally good research for a PhD graduate from your department, they will both have the networks to recommend you for a postdoc.

For positions that are not research-focused, it really doesn't matter what their reputation is, because the people hiring for non-research positions are generally out of the network anyway (and even if they are not, they won't be hiring based on whether they know your advisor or not).

Work with the person you like working with better and whose projects interest you more.