i heard if you buy the semi newer sim free iPhone 6 from the apple store now a days, it will be whitelisted (meaning it will work on) Verizon, Sprint, T-Mobile, AT&T, etc.
this means that you can use this iPhone to switch from Verizon to Sprint and back.
this makes it highly desirable when it comes to selling it.
when the launch day iPhone 6 came out it was not whitelisted to sprint or verizon, there was a loop hole where the t-mobile iPhone 6 would work on verizon, but it was not official like it is now.
if you buy the phone from verizon, you won't be able to use it on sprint. because sprint will see the serial # as a non sprint phone.
i would buy the sim free iPhone 6 directly from the apple store. go to sprints website / a sprint store, to see if they say the iPhone is compatible, to make sure, i am sure sprint will say its supported.
even if you never want to go to sprint , the ability for someone else to use it on sprint in the future would be a big selling point.
However:
now if we are talking about a subsidized and discounted verizon iPhone where you get a discount for a 2 years contract. i don't know of any advantages at the verizon store versus the apple store. , apple store probably would take paypal and have different financing offers, and the verizon iPhone you would get would not be whitelisted to work on sprint.