Coverage won't be the same, even in large metropolitan areas. T-Mobile USA simple has fewer cellular towers than AT&T, fewer LTE transmitters, and more coverage gaps. I see it all the time in the Greater SF Bay Area.
And forget coverage maps, they aren't detailed enough to show specific coverage gaps.
The first thing to do in selecting a cellular service is to identify which carriers best service the places you frequent the most (home, work, maybe school). For the latter two, ask the company or school IT department; they will definitely know. At home? Well, invite some friends over, then query them about cellular reception on various services from your house. You don't want to guess, just be methodical.
Once you understand the pros and cons of each service's coverage in the places you frequent the most, you have a better chance of making an informed decision about which provider to use.
I did the research myself, and decided to compromise and use a service with spottier and slower coverage, mostly because it still works adequately in my primary usage locations, plus the price was right. My coverage may be rather spotty when I travel, but I mostly visit metropolitan areas, so that's less of an issue.
(Disclaimer: I have used both AT&T and T-Mobile USA prepaid services as well as some prepaid MVNOs which connect to those two services' cellular towers. I am currently a T-Mobile USA prepaid user.)