One possible plus for the DL alliance is that you can redeem awards on CO and NW also. When trying to put together a difficult award redemption it can help having this choice.
We were trying to fly to Hawaii for spring break and could never find a complete roundtrip on AA, DL, CO or NW. But we were able to mix and match and piece together a trip using DL, CO and NW segments. It was cumbersome to do the research on all 3 websites and the routing was odd but it worked.