Of course you need to research what the "true cost" to get that deal will be. To get that deal at Sprint you must purchase a "non discounted phone" which happens to be $650 for the 16 GB iPhone 6. And it applies to "base monthly service plan only". Yes, they have small print in their offers, just like any other carrier.
Keep in mind your bill would most likely be cut considerably if you stay with Verizon and purchase a "non discounted phone" when your contract ends.