Best deal requires some money up front and going to a different operator (or mobile virtual network operator), but the money saved over the contract life with AT&T should more than make up for it.
With AT&T:
99.99 Phone (if you get that $100 trade in mentioned)
36.99 Activation fee
2879.76 (119.99 x 24) (individual plan for unlimited minutes and text with 3GB data for 24 months, not including taxes)
$3016.74 Total
Straight Talk:
764.99 Phone (unlocked)
$50 Straight Talk SIM for AT&T network(can get micro-SIM or cut a standard, need to get from eBay or other party as Straight Talk can't sell them right now, prices are varying)
1200 (50x24) Straight Talk unlimited voice, text and data (used $50 instead of advertised $45 due to taxes or such, so it will vary I just went to a high number
$2014.99 Total
Since both are on the AT&T network for the actual service there is little to no difference in coverage. From what I've read the only time you may notice a difference is if you were to be in a roaming area on someone else's network that has a roaming deal with AT&T. With Straight Talk you only get service directly from AT&T when using the AT&T network Straight Talk SIM. I've not found a place I normally go without AT&T though.
Heck, with that difference you could even update annually and still be a little better on the price.