I've been using older model iphones for years, but the problem is that the plus size is too large to feel comfortable in my hand, and the smaller size is too small for me to see now and type on comfortably. I upgraded this summer to the iphone X, but instead of new, bought an excellent-condition one through Visible wireless, a Verizon subsidiary, and then simply didn't establish service, but used it on an ATT prepaid plan. It's really a perfect size and weight, amazingly fast, and the face ID is also amazing to use and gets rid of bezels and buttons. I would think that as soon as the new iphone is announced next week, used iphone X will be even cheaper. A good place to look is swappa.com, which requires detailed photos and descriptions, with paypal purchase protection and ratings. Another option for those with comcast is to go for xfinity mobile service, which is $45 for unlimited data and is giving a $250 cash card for those buying new phones (so an iphone XR would be $750 minus $250 cash card).
The iphone 6/6 plus also lacks a newer network frequency band for T-Mobile, so you'll get inferior coverage. If anyone is looking for a serious deal plan for a couple, T-Mobile has an over-55 plan, with two lines for $70 a month, everything included, even Canada/Mexico data coverage (my sister just switched to that plan, after being robbed by all of ATT's price increases and taxes and fees for years).
For those looking to trade in an old phone, I found the company who runs the Apple trade-in program very strict about condition and even refusing phones I sent them, and the credit can only be used for Apple stores. A much better trade-in site I found is decluttr.com, which was much better about accepting phones and paid a decent price (and resells them, another place to look for a newer used phone). I think I also had one old iphone I just took to an ecoATM phone kiosk at the mall and got some cash for it (using a bonus code from retailmenot.com).