IMHO, the only major advantage is price. I'm currently on the $30 Walmart plan with my M8 that's $55 cheaper than the discounted AT&T plan I'm getting rid of. That works out to a new flagship phone every year if I want.
Two minor advantages: speed and Wifi calling. So far in my 3 months on T-Mobile, when I get LTE on T-Mobile, it is many times faster than AT&T, sometimes an order of magnitude faster (say 3mbps vs 30mbps on T-Mobile!). The caveat is that I get LTE in fewer locations than AT&T and sometimes in the middle of big stores like Target, I lose reception altogether.
T-Mobile M8 has Wifi calling which theoretically works better if you have spotty cell reception. Also supposedly allows free calls from overseas (haven't tried myself yet). In reality, Wifi calling hasn't worked for me too often, sometimes it just makes a call then hangs up. It also interferes with ISIS mobile payment because it deactivates the SIM, rendering the security feature unusable.
Just reread your message and Wifi calling might be irrelevant anyway.
If you want to try it out and you have access to an unlocked phone, one option is to get the $30 Walmart plan and see how the coverage goes for you.