I switched to NW earlier in the year,but found myself back with Delta mid-year.The main reason is TIME. NW is much ,much, much ,much better than Delta for elites.They,however often take up too much of my time connecting.The new allaince opens some new possibilities.But I admit that although we in ATL complain about being "held captive",we've really been spoiled by so many direct flights. By the way,I agree that AirTran is a nice alternative (when available).
The problem with going with AirTran,it can make it more difficult to make higher level elite with your primary airline.