I was positively thrilled with my S3. When I got a chance to upgrade early to a Note II, I did, and as I use a phone mostly as a small tablet, it was a great move.
I don’t think the S4 brings a lot to the table unless you’re a complete latest’n’greatest junkie. I wouldn’t pay the premium launch prices for it. The S3 will likely make you berry, berry happy and will not be outdated in two years.
You might be in a great position to make the most cost-effective move you can - buy an S3 outright and negotiate the best non-contract plan you can get. You can save hundreds or even thousands over the life of a 2-year contract with a subsidized phone.