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AA is resuming ATL-LGA nonstop service on regional jets as of January, 2015. If you can find low-tier award seats on those flights, they would cost you only 7,500 BA Avios one way in Coach, with no extra charge for a close-in award booking, and minuscule fees for canceling an award flight and re-depositing the Avios, if you did so at least 24 hours before departure.
However, AA will not have nearly the number of total seats that DL has on the ATL-NYC route. But that's not to say how many miles DL would want for an award seat on any of those flights.