I was on AT&T for some 10 years because it and Verizon were my only options due to coverage. I had tried Tmo about 5 years ago but the coverage was fairly well terrible at that time and went back to AT&T. A year ago I had been hearing that the coverage was much better and tried again. This time the coverage was workable, not nearly as good as being on the duopoly but much better than before. As I said workable for me based on the trade-offs of being on either carrier. I will say that Tmo's coverage is getting better, there are now a few places I didn't have coverage before that I do now.
For me the advantages of tmo are... Costs less. I now spend less per month but have much more in my bag of goodies. Which brings me to data, a couple gigs on AT&T versus unlimited on Tmo and for me that unlimited has worked out to be just that, I have never been throttled that I could tell and use about 20 gigs a month. Free roaming overseas, huge for me. I travel a lot and I used to get a sim card in the local area and run that. Now I just turn airplane mode off and the phone works without having to pass out temporary numbers to friends and relatives, get the sim card in the first place, waste money on extra that I wasn't able to use during the trip. Its also nice to be able to make free or low cost calls to overseas from the US which I do enough of to make it a value add. While Tmo says the data overseas will be 2G more often than not I find the phone on 3G which isn't what I want but good enough. No locked bootloaders so at times their phones can be cheaper than the unlocked international versions I had been buying before. Wifi calling will soon be a non issue since everyone will have it on their carrier but it has been something I've used without an available phone signal while away.
The advantages of AT&T for me were coverage. Nothing else.
About Fi, its a good deal if you use less data but can get expensive if you use a lot of data. I need to tether at times and for me it would be much more expensive.