the iPhone 7 sim free will not be the same exact thing as buying an iphone 7 verizon, because of the imei # of the phone. if you buy the Verizon version, its serial number may not be listed as compatible on AT&T , sprint or t-mobile. if you buy the sim free version, then the serial number will probably be listed as compatible on AT&T, sprint or t-mobile.
i pre ordered two iPhones on launch day (the 6 plus and the 6s plus ) and it took anywhere from 40 days to a year for AT&T or sprint or Verizon to say my launch day phone was a compatible device.
if they say its not a compatible device then you can't order a new line of service. however, using a pulled sim card from another activated phone may work
if they say its not a compatible device, they will not sell you a sim card.
the bottom line is i believe on my experience is that if you wait for the sim free version, you will receive a better serial number then someone who bought a Verizon.
i haven't bought a iPhone 7 yet. so i don't have any imei numbers to check, so this post is based on the last 2 years but not necessarily this model.
if someone out there with a verizon iphone could please have imei numbers checked with the other 3 carriers , and if all 4 agree that the device is compatible, then my post is wrong and you might as well just buy the verizon model today
with the iphone 6 it took about a year for verizon to agree to activate the t-mobile model
with the iPhone 6s , it took about a year for AT&T to agree to activate the a1688 model.
with the iphone 6s it took a few months to get a good result from sprint on my t-mobile model
with sprint it took about 45 days to get them to like my serial number of my a1688 , because it was a launch day model
for a whole year AT&T was only activating the a1633/a634 because that model had band 30. but a couple weeks before the iphone 7 launch, the 1633/1634 requirement was dropped