I was AA PLAT and DL GOLD at the same time. I prefer AA for several reasons:
1. Able to upgrade almost any fare. (and not to hard to clear for int'l flights).
2. Lounge Access on International flights
3. Lie/Flat seats in J and F.

Why not mid/top tier on 3?
you fly 150-250 on delta and other on NW, Why not Platinum for Delta, and EXP for American. And maybe Gold/Plat for NW.