iPhones don't usually depreciate very much. Probably no more than $100 from the used price.
I know the US has healthy respect and pricing for 2nd hand good, from personal experience but looking at simple outright costs opposed to special offers/deals that often have hidden costs and lock in T&C's
If we look at S7 you can buy an Apple S7 or S7+ for around $170 cheaper than release date but to be comparable with your release date IP7 you would have to of bought AC at $99 to offer same minimum one year warranty
So now your down to $270 and your device has to be in mint condition, less fee's etc for selling on ebay say ~35% down but only if you sell 6 months before next release, so what to use for 6 months ?
Apple current trade in price (worst option but easy) is IP7 $215 and IP7+ $290 so at least 50% loss or more if you factor AC in
Typical other trade ins fare better (Amazon, Walmart, Gazelle) where you are only likely to drop 35-40% (excluding AC cost) but you do not always get cash back and there could be other catches
Sure YMMV as you can find a special deal that suits you but there is often strings attached that are hard to value (hidden cost in carriers pricing) that make the trade in seem good on face value. If you wait until new release date you will probably loose another $100 due to everyone trading/selling at the same time
I would say Samsung are a good 10-15% worse than Apple from release date and a bit less if you wait a little while from release
Personally I always have a Samsung sim free no contract current release and a previous year as back up with no extended warranty. I just trade in the oldest for the new each year direct with Samsung trade in ~55% loss. So maybe I could of been $80 or even $100 better off with an iPhone but buying on release date you normally get an additional bundle with Samsung. This year it was a free wireless charging stand worth $80 and 15% off accessories