Traffic shouldn't be a concern for you, driving on a Sunday. I'd just go with whichever one is closest to your preference. Between Baltimore and DC, there's really not much difference between 95 and 295, except that 95 has more lanes. Once you get to DC itself, though, 295 offers the most direct route through the city. Unless there's an accident, though, I think you'd be fine on either road on a Sunday.
Once in the Anacostia area on 295, you have the chance to get on 395 if you like. This turns into 95 south of DC once you clear 495. 395 is horribly congested on weekdays (I used to live right off 395), but not so bad on weekends. Look for signs that say something like 695/395 to get onto this. Otherwise you can stay on 295 and then go west on 495 for a bit to get on 395.