A DL/NW merger would probably be easier for DL to pull off on DL's terms, but would require a lot of work to integrate.

The two airlines have different fleets, different strategies, and different cultures.

The biggest benefits to DL would be the SEA and MSP presence, as well as Asian landing slots.

However, NW's hub and spoke approach to Asia does not fit with DL's point-to-point approach to international flights.

The one airline merger which strikes me as new-perfect from a route and fleet integration standpoint would be UA/CO.

From a strategy standpoint it would be DL/CO.