Well I'm on my second year. Bottom line is, if you pay outright (let's just use $900 as an example because depending on gb, it could be around that price), you'll spend $900 plus tax of $90 (in Louisiana it's 10%, so figure for your state). If you do the upgrade plan, you'll initially pay your $90 (or whatever in your state) tax PLUS one month of your payment plan, so $130.
So initially:
$990 or $130
Now with the payment plan you'll have 11 payments at $40 before the next phone comes out, so $440 more.
At this point you're out either $990 (and you own phone) or $570 (payment plan).
So now you have the option of selling the phone you own, or turning in the phone you've paid $570 on and getting the next one. You'll break even if you can sell your 1 yo phone for at least $420. Anything over that would be profit.
You'd also have the option of keeping the one you have on the payment plan, paying 12 more payments ($440) and owning it, at which point you'd have spent a bit more money than original purchase price, but Apple Care would have been included in the payment plan (if you purchased AC with the out of pocket phone, you'd have to add that to your total and I think your cost would basically be the same).
SOOOO, long story short, cost is pretty much the same - it's really about whether you want to put the money out up front or not. The payment plan works for me because I'm getting a new phone every year, there's no interest, no hassle of selling the old phone, Apple Care is already included and it just automatically gets charged to my credit or debit card monthly - pretty easy.