What is best transit to Metro from BWI and back? My wife and I are staying in Dupont Circle and plan on using Metro as much as possible for the week we are there for Cherry Blossum Festival.

There are several options, but if it's a weekday I would recommend taking a MARC train from the BWI rail station to Union Station in DC (if it's a weekend you can take Amtrak from BWI to Union Station). From Union Station you can either take the Metro (Red Line) to Dupont Circle or take a cab.

There is also the option of taking a bus from BWI to the (I believe) Greenbelt metro station and then take the metro to Dupont. Although this is cheaper I would recommend taking the train.

There's also the shared van option ($30/person one way, with reductions for round trip or more than one person to a destination), for times when MARC is not running.
The blue van's advantage is that you go from BWI baggage claim to your DC destination without having to change from shuttle to train to Metro. The disadvantage is that you could be the last drop-off (and may get antsy while they go to five other hotels before yours).

Thanks for the info.; after addtl. research, it looks like Amtrack to Union Sta., Redline to Dupont is the fastest. Bus to Greenbelt & switch to Redline at Fort Tolten looks to be more time, altho cheaper.

I will be traveling on Sat., so MARC is not a option.

I will be traveling on Sat., into BWI at 5:40pm; how is traffic, for the bus or the van. I will also be returning on following Sat.

Traffic on saturday to BWI shouldn't be a problem. Leave yourself some time of course, in case there's an accident or something, but you should be OK.