Why does T-Mobile prefer AT&T roaming in Maine over US Cellular ?

I’ve been using Canadian carriers for ~15 years.  T-Mobile speeds are good, and service is generally good (Canadians pay through the nose for service, so it’s typically under utilitized).  You’ll primarily be on Bell/Telus from my trips on T-Mobile.  Cities may pick others as well.

Rogers typically doesn’t allow for selection (you might get it in NW Ontario).

I’ve hit ~200Mbps on LTE (Samsung doesn’t have compatible n5 band on S21).