They're rather different.
AT&T's coverage drops and water damage like apple care, but it also covers your phone being lost or stolen. However, last time I checked, the deductible was significantly higher than $50 per incident.
Applecare Plus actually gives you coverage for two years. But as far as I know doesn't cover lost or stolen devices. Your also only allowed two replacements at the $50 level (not sure what happens after that point).
A good advantage Applecare has, however, is on the spot replacement if you take it to a store. I think AT&T is only a next-day guarantee.
Honestly, while I've been a purchaser of Applecare in the past, I'm starting to think that both are a racket. It seems that most people are keeping their iPhones for only one years these days, and the standard warranty covers that time period (though not for drops and water damage). AT&T's insurance might be the better deal then, but I'd double-check what the deductible is.