If you frequently fly DCA-ORD, I would highly advise that you stick with UA or AA.

Yes, you have the shiny DL AMEX. However, is that really worth the extra time connecting through MSP/ATL/DTW/CVG/MEM/LGA? In addition, it's been well documented that DL miles are a bit more difficult to redeem than UA miles. If your end goal is redemption, you might want to rethink the wisdom of this switch. You will have more miles right now, yes, but who's to say 6 months from now Chase won't allow you to get the UA Explorer card? While domestic upgrades are slightly easier on DL, there are lots of CVG-ORDs and MEM/CVG-MKEs on CRJs with no F.

I have flown DCA-hub-ORD before, but as this is not one of my main routes I was totally fine with this. The hub connection just adds an exceptional amount of complexity considering that there are perfectly good ORD nonstops operating on UA/AA.

Each airline has a different set of hubs and routes. Sometimes a certain airline just won't have the tools you need for the job. If this is for business travel where speed is more important than anyone's mileage balance, I don't know if I could justify the connections required.