IMO, based on just those 2 choices, I think a 16GB iPhone with AppleCare+ will hold a better resale value, assuming that you also seek a prorated refund on the unused portion of your AppleCare+ coverage.
My thinking is based solely on seeing previous re-sales & coming to the conclusion that base (or lowest price) models of any Apple hardware products are the ones that will cause you to lose the least amount of money compared to what you paid when you bought them brand new.