Subfolder or URL parameter?

I'll make this quick and simple. Let's say you have a business located in several cities. You've built individual pages for each city (linked to from a master list of your locations).

For SEO purposes is it better to have the URL be a subfolder, or a parameter off of the home page URL:

https://www.mysite.com/dallas which is essentially https://www.mysite.com/dallas/index.php

or

http://www.mysite.com/?city=dallas which is essentially https://www.mysite.com/index.php?city=dallas

I think John Mueller said not to create city specific landing pages because they could be construed as doorway pages, so I think a url parameter would be better, right?