What are you doing in DC and what times do you need to be there?

As noted above, the MARC doesn't run terribly frequently on the weekends. That can also be true of metro--there's often a lot of track work on the weekends.

If it were me, I'd drive into DC and either park in a lot or park somewhere closer in. Since it's the weekend, traffic will not be so miserable. Just where to park depends on where you are going (and also what you want to pay for convenience vs. keeping your costs down.)