Do you fly full fare or deep discount? It makes a difference.
If you fly full fare or higher fare (non-L,U), I think DL might service you better.
Reasons:
1 Less RJ.
2. Very good upgrade percentage.
3. Free Crown Room membership (as PM)
4. You can also connect thought Memphis, Detroit on NW hub and CLE, IAH on CO hub. Although you won't get upgrade on CO or NW, their policy might change after CO/NW join Sky Team.
5. You can redeem on SQ or Virgin Atlantic to Europe and Asian.
AA is good. But if you have to fly RJ, I will stay away from it.