I am interested on thoughts about URL structure for city landing pages.
Essentially best practice and reasons for implementing format (a) over (b):
(a) example.com/chicago
(b) example.com/?_city=chicago
Using the path instead of the query component is typically considered to be more user-friendly. So go with (a):
/chicago
If you want to represent (in the URL) that it’s about cities (what (b) does), you can do this in the path, too:
/cities/chicago
This allows you to provide browsable URLs, i.e., offering a list of all cities at:
/cities