I think you have to think of it in terms of total cost with whatever contract you're on.
Last year on 5S launch day I bought a 64GB for £719 direct from Apple and have had a £15 a month sim only contract for it. Last week I sold it for £430 on Ebay (with a couple of cases so lets say £400 was the phone). Take fees off the £400 and I'm left with £340 in my pocket:
£719 +
£15 x12 =
£899
- £340 (resale income)
= £559 (£46.58 a month as a whole cost).
So either way, when you compare it to the same carriers phone and sim contact I would have been paying a £149 upfront cost and then ~£50 a month (for 2 years not just the one year before I could upgrade).
If you then think what might have happened if I had kept the 5S for another year... Say I would net £200 in the end for the phone at that point (basing this on what i got for my then two year old 4s a year ago when i sold that).
£719 +
£15 x24 =
£1079
- £200 (resale income)
= £879 (£36.63 a month as a whole 2 year cost).
So whilst I get less for the handset after two years (though based on my experience a lower proportion of the start value is lost in year two than year one) the total monthly cost has reduced by more than 20%. So from that point of view keeping the handset for two years would be better value.
But it really depends if you're buying the phone outright or on contract (most now seem to be 2 year ones anyway) and how much you want the latest phone each year, (for me its worth the extra monthly cost to sell after a year and get the new phone).
Thank you so much.
I buy my phones outright from Apple Store and get a cheap sim deal £10 per month. Which gives me all the minds, texts and data I need.
I kept my iPhone 4s for 3 years and sold it on fleabay for £140.
I will probably keep my 6 for 2 years now and sell a few weeks before new iPhone is released to get max money for it.