while AA usually has better award availability, I would actually say to transfer them to DL. The reason is:
you can currently churn credit cards for 25k bonus miles on sign up with citi and get uber AA miles. With DL, you need to get an AMEX and it's the only card right now to get miles on that airline. OP has an AMEX and that is the wisely chosen SPG AMEX.
NWA will soon go away and if OP has an NWA visa like I had, your miles will eventually become DL miles but if you want more in there, you need to either get a new credit card or transfer from SPG.
just my thoughts. (plus, I was fortunate to have registered before that DL promo closed so transferring in SPG will give me more bonus points this month!)

MM