If you visit DL on either a Saturday or Sunday, you will run into crowds. I think that either day will have the same amount of crowds.
You don't have to worry about that problem at DCA. Any day you visit DCA, you won't run into unbearably long lines so you're flexible in that regard.
What I would do is visit DL on a Saturday. DL is usually open later on Saturdays than on Sundays so even though it will be crowded, you will have an extra hour or two to get more stuff done. Then on Sunday, take a break from DL and see DCA. You can even sleep in from the long night before and arrive around 11PM or 12 noon and still get everything in. DCA doesn't have as many attractions as DL so you'll find the day to be a relaxing change of pace from the day before at DL. If you can grab fast passes for the e-ticket rides at DCA, then you can use your time more wisely by catching a show or seeing an exhibit. You may even finish your day early and get to bed early enough or you can repeat your favorite attractions as often as you like. I hear that after the parade, DCA clears up considerably and there is hardly a wait around 7pm for everything.
Then on Monday, go back to DL and catch everything that you missed on Saturday and repeat the stuff that you liked.