I live in Crystal City and have taken both the Washington Flyer bus ->West Falls Church -> metro route as well as the 5A bus.

If you've got minimal luggage, they're each decent options with their own pros and cons.

It's nice that the Wash Flyer bus leaves more frequently and that the ticket purchase and boarding procedures are pretty streamlined. Depending on what time of day you arrive, you might not be able to find a seat on the Orange line train from West Falls Church to Rosslyn (the trains are overcrowded with commuters). And it can be hassle to change trains when it's crowded at Rosslyn.

The 5A bus doesn't always follow its printed schedule, so if time is a factor for you, it might not be the best option. Also, traffic on 66 to Rosslyn is pretty congested in the mornings and buses get caught in that, too. On the pro side, it's nice to not have to change trains and you can't beat the price, even on non-code red days.