I would say that if you are a high-volume, high-spend flyer more focused on upgrades than redemption value, DL makes sense.

If awards, particularly international awards, matter to you, and you care that DL SkyMiles are generally worth 40 to 60 percent less than other programs' miles, I would say it makes more sense to stay away from DL, or at least put your DL segments to the AS program.