I called VZW today to try to figure out how much all the options would cost. Currently have a 64 GB 6+, one of 4 phones sharing 13 GB of data (which we've never exceeded). We're grandfathered in to the subsidized phone every 2 years plan; I got the 4 when iPhones finally were available with VZW, and have upgraded to the 5 and the the 6+ when the came out. The 6+ makes the comparison more apples and oranges (sorry), so I'll use the 6 buy/sell prices here. I am also going to compare the middle storage capacities.
Paying the full retail price for the the standard size iPhone is $749, but the VZW "Line Access Fee" drops $25, from $40/month to $15 month. In addition, the 64 GB 6 is selling for an average of $528 on Swappa currently, while the 32 GB (middle storage capacity) 5S is selling for $335 on average. Based on those figures:
Upgrade yearly out of pocket: $750 for the phone + $180/year Line Access Fee - $500 for the sale of the previous year phone (rounding down to account for shipping and always getting less that advertised). That comes out to $430 a year for the phone and Line access fees, $860 for the two years.
Upgrade every two years out of pocket: $750 for the phone (only paid once in this scenario) + $360 for two years of Line Access fees - $300 for the sale of a two-year-old iPhone at time of upgrade (again rounding down) = $810 for the two years.
Upgrade every two years on contract: $299 for the subsidized phone + $960 for the Line Access Fee - $300 for the sale of a two-year-old iPhone = $959 for the two years
Upgrade every year with Apple's Upgrade Plan: $439 a year (12 x $36.58 installment) + $180 for the Line Access Fee. That comes out to $619 a year, or $1238 every two years. You do get perpetual AppleCare+ in this scenario; it would be $375 a year to pay installments to VZW (obviously without AppleCare+), which works out to $1160 every two years.
tl;dr: The installment plans will cost you a $150-200 premium every year to not have to deal with the headaches around selling/shipping. The subsidized phones will cost you about $100-150 more than buying them at full freight, and for $25 extra a year, I am strongly considering upgrading every September.