Woodson has good demographics (almost no townhouses, few apartments in-boundary- so a reasonably wealthy, mostly white population) so it has good test scores. It is also closer-in to DC, Tysons and Metro than some of the other schools in Fairfax, and you can get a decent home in the boundary for a relatively decent price. (for DC. nearly anywhere else it would be insane.)
According to my friends who went there, it is also kind of a pressure cooker. They're all very smart, successful people but they say that it could be a tough place to go to school - their peer group was pretty driven. I think it's smaller than Oakton, which has a rep for being good, but really big.
A friend of mine teaches at Marshall, and I've heard some really good things. Marshall was kind of marginal, middle-of-the-road for Fairfax for a long time, and the last few years it has really climbed up the rankings of the school district. It's up there with the other M's now - Madison and McLean.

Your kids should be fine. I'd be more worried about Tysons traffic if I were you than Marshall.