I'm on Fi and my dad is on RW.
I think it depends on your usage patterns (data), and your T-Mobile coverage.
Side note: Fi is adding US Cellular to the network soon too.
https://android.googleblog.com/2016/06/more-speed-and-coverage-with-us.html
Does Fi have the same problems as RW? Like, I get messages for the old Sprint number, pretty annoying.