I've had my iPhone 4 since 2010 and it has given me good service but I find that the manufacturer is not upgrading the operating system any more and I expect to find in future there will therefore be more and more applications that are incompatible with my phone. Another shortcoming with the model I own is that the battery life is not that good. I know from anecdotal evidence that more recent models have much greater storage capacity in their batteries.
The woman at the counter gave me a sheet of paper that details what's included in each package, and she wrote the monthly charge on the paper for each package. I asked her to give me prices for the phone with the biggest storage capacity, which is the one with 128GB of storage. I calculated the total cost over the lifetime of the contract - a contract lifetime of 24 months - and then deducted the cost of the same iPhone 6S as priced by Apple, and it came out to a difference that was about the same as what I currently pay the service provider per month. This tells me that the charge for the purchase of the phone from the service provider is realistic.
I plan when I do upgrade the phone to donate the old phone to a friend who lives on a restricted income and has never been able to afford an iPhone, so right there will be another good outcome from the upgrade, as well as it giving me access to a superior device that I can continue to use for the next 5 or 6 years.