I had T-mobile before switching to Project Fi. Overall, I found reception with Fi better than T-mobile but that will depend on where you are. The costs ended up being ~$10 less/month using Fi for me (one line) on average. Any month where I wasn’t traveling I would pay ~$30.
I love the Pixel series phones. Fi uses an e-Sim in the Pixel which leaves my sim card slot free for another card (handy if I’m traveling and want a cheap sim card for local phone service without roaming). I can switch between the sim and the e-sim easily. Fi does not charge extra for data roaming or international texting, but does charge for phone calls placed from outside the US.