I don't think it'll make a difference with a trade in.
I was really steamed back when the 6s came out and websites like walmart, is it worth more, gazelle , etc. were low balling and giving you a lower price because your phone was "an unlocked AT&T" and not a "verizon" even though it was the same exact phone. they were knocking off $$$ off your phone trade inbecause you said it was a sprint one. and once sprint is unlocked its the same damn phone as everyone else
since those days, they mostly wised up and now they offer a $20 +/- difference between the different flavors of the 6
back when the iPhone 6s came out, the only company that deals with trade ins that was giving you the same exact price of any phone, regardless of carrier was apple itself
it looks to me that the 5s is CDMA+GSM and therefor once you unlock AT&T it becomes a verizon phone.
it's really moot to argue if AT&T gives you the unlock for free
here are my " is it worth more " quotes from 9/21/15
unlocked 481
tmo 376
verizon 476
AT&T 456
other carrier 256
sprint 331
once the phone was unlocked on the particular day it was worth $481 regardless of carrier
these are the quotes from gazelle
Gazelle’s price quotes as of 9/4/15
iphone 6 plus 128 gb , T-mobile UNLOCKED
AT&T $346 Sprint $361 T-mobile $383 Verizon $476 Unlocked $365