To me, it looks like the $129 price of the 2-year AppleCare+ is spread evenly across the 24 months. So if you upgrade yearly, you will have only end up paying for 1/2 of the $129 AppleCare+ price.
The interesting thing about Apple's Yearly Upgrade plan compared to AT&T Next is that Apple divides the price of the phone over 24 months.
On AT&T Next, to upgrade yearly, you have to be on their Next 18 plan, which has the price of the phone divided over 18 payments.
This makes Apple's plan a little less expensive per month, even though it has the extra cost of AppleCare+ added in.
If Apple didn't require going into a retail store to do their yearly upgrade plan, I'd do it.