Only the Republic Wireless Moto X has native WiFi calling, AFAIK.
I use them and it has worked well for me. Speed and coverage are entirely dependent on the Sprint service in your area.
Where I live, Sprint recently upgraded to 4G, and speeds are in the 5-10mbps range, which is perfectly acceptable for me. If you have any friends on Sprint, you could ask them what speeds they experience. Additionally, Republic wireless offers a 30-day money back guarantee.
If you would like to try them out, I'll give you a referral link to save you $20 off your bill. You HAVE to use a Republic Wireless Moto X (or Moto G) on RW though. You cannot bring your own device (because of the custom rom with integrated WiFi calling).
EDIT: Here is the referral link, if you decide to try them: http://rwshar.es/28KK (This will save you $20, and I get credit as well. Win-Win )
Get RW's Moto X - I'm on their $10 a month plan and it's excellent. If you travel outside the US a lot you can only use wifi - you can't pop in a local sim.
EDIT - Moto X starts $299 black/white 16GB, $349 MotoMaker 16GB, $374 MotoMaker 16GB Wood, $399 MotoMaker 32GB, $424 MotoMaker 32GB Wood. Moto G starts at $149 8GB, $179 16GB