As you're already on T-Mobile, the big question you need to ask yourself is how much data you use. If you don't use a lot of data, Fi can be a good deal. I'm on WiFi most of the time, and cache everything I can and RARELY break $35/month on my bill (which means I often don't use more than 500-600 MB in a month).
I've HEARD that even though Fi is selling the devices they still don't support the network switching. If T-Mobile works well for you now, Fi will be the same as the "default" network is T-Mobile. There have also been some comments surrounding visual voicemail not working as expected, I don't have one of those Samsung devices so I can't speak to that from personal experience.
If you want "free" hotspot (it uses the same data pool so you still pay for data) AND the option to add data-only SIM cards for additional data-capable devices PLUS have international service available, Fi could be a fit for you.