Well, Sprint is in the news now for the best coverage at O'Hare. But your mileage may vary at other airports. My understanding is that nothing at all has been upgraded at Sky Harbor in Phoenix and right now we sit at around 59% completion of Network Vision for the PHX market.
Sprint has a policy of never activating phones it did not sell or that were never intended to be used on their network. I have heard a report of one person getting a Verizon iPhone activated on Sprint, but IDK. May want to check it out.
That said, Sprint's frequencies are 1900mhz, 2.5Ghz and 800mhz. I don't believe the Verizon iPhone 5 series operates on the same LTE bands. So, even if you manage to get it activated it may not work for LTE.
Lastly, Chicago was one of the first Sprint markets to get upgraded with Network Vision. Sprint used it as a test market to polish the rollout plans for all their other markets. All the pain of getting the old Samsung equipment and the new network equipment to work together has been alleviated and Chicago is now one of the more mature markets Sprint has. You shouldn't have any real problems there if you have a phone that can use Sprint's frequencies.
Oh yeah, the iPhone 6 and 6+ are the only iPhones that can take advantage of Spark. The 5c and 5s can handle 1900mhz and 800mhz, but not 2.5 Ghz and the iPhone 5 can only handle 1900mhz for LTE and 800mhz for voice.
The iPhone 6 and 6+ will not do VoLTE as Sprint has not launched anything for that yet.
Sprint would not active my Iphone 5. I would trade it into them, for $230, to be applied to my bill. This would be done, 14 days after I get a sprint iphone 6, to try out in Chicago (I am in Detroit right now for the weekend)....then if I was happy with sprint, keep the 6, and trade in the 5. As for VoLTE and wifi calling, I think it's just a matter of time they enable it for iphone 6 users (but given how time takes so long with sprint, it may not happen soon?)