TLDR; if you're serious about having iPhone long term, and want to upgrade every year, the upgrade programme is slightly better value. Plus gets you Applecare+.
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There is another way of looking at this longer-term. Let's say, like me, you want the iPhone 7 128GB.
CREDIT CARD:
You buy it outright on a 0% credit card for
£699. No Applecare. You pay off at £58.25 per month before you want to upgrade this time next year.
You then sell the iPhone 7 for
£400, and purchase the iPhone 7S for
£699 (assuming prices stay the same for the sake of this example). Everything included, you've got a new iPhone til Sept 2018 and you're
£998 down.
UPGRADE PROGRAMME:
You pay
£45 up front and then
£38.45 for 11 months. That's
£467.95. Includes Applecare+. You then choose to upgrade to the iPhone 7S which costs another
£45. You'll then have another 12 payments (including your September one) through to the next upgrade, worth
£506.45. This also takes you through to Sept 2018 with a new iPhone. This time, though, you're only
£974.35 down.
This cycle would continue until you wanted to opt out of having an iPhone. When - actually, if - this happened you'd be far better off owning your iPhone and selling it to opt out. Because with the upgrade programme, you'd have to wait a further 8 months before you own the phone.
If you think you'll be sticking with iPhone for some time, though, it looks like the upgrade programme is probably worth it. Particularly if you want the added benefit of Applecare+. I worked this out pretty quickly, so do please let me know if I've taken any arithmetical wrong turns.