Need advice on subdomain versus folders

Hello, I’ve been reading up on this for a couple of days but I would still love some advice on whether to use domains or folders.

Here’s what I’ll be doing…the site is intended to serve ads and posts based on geotargeting so, for example, Salt Lake will get certain content and Kansas city will get other content (posts and ads). There will be some content that is consistent regardless of the city. I don’t necessarily plan to have multiple templates but if a client asked for a custom skin, I would like to be able to do that (or know the plan for making that happen if it comes up over time).

There won’t be any other admins or users setting up their own blogs. We’ll administer this in-house but I will want to set up either https://www.domain.com/salt_lake or https://www.saltlake.domain.com. I would really appreciate any feedback on the best direction to take. Love WordPress and excited to use MU if I even need it for this purpose!