I think we have the same AT&T prepaid plan. I forgot it was 1.5 GB since I don't really use that much. I only signed up for 2GB with Fi because I knew they'd give back whatever I don't use, and at its base its still cheaper than AT&T.
I'm not sure if you've looked at it, but you're required to use one of the two new nexus phones, either the 5x or the 6p. The 5x is an awesome phone and coming from a Fire phone, it's nice to have a device I can do whatever I want on and install any app I need without circumventing the phone's OS.
The wife's never been in a situation where the phone has had to switch from cell service to wifi while on a call, so I'm not sure if there's a delay. The wifi calling works perfectly fine though, no issues there.