Here's my $.02:
MD: Better road infrastructure, slightly more lenient cops, less offensive speed zones, does not require annual vehicle safety inspections, unbelievably expensive homes (depending on where you choose to live, could pay $400 - 600+ for a modest townhome, there are the seven figure townhomes as well), state taxes are ever so slightly higher, but not so as to make a significant difference (IMO), no personal property tax on vehicles, biotech heavy.
VA: major traffic on many of it's primary arteries (primary reason we moved from VA to MD), housing is quite expensive here too, personal property tax on vehicles (though this is being phased out - where are the lost dollars going to come from?), incredibly aggressive local cops, especially in Loudon County, lots of artificially low speed limits, car insurance is slightly less expensive (again, modest difference), lots of high tech companies (especially in Tysons and Reston areas of Fairfax County).
Overall, I really like both areas. We live in Montgomery County, MD and they have built a good deal of really nice shopping areas right by us, so that's wonderful. It's relatively easy to get around (I-270 often sux, but I-66 in VA almost always sux). I loved living in Reston with the high-rise office buildings, etc, it has a very modern feel. However, living in Reston got to be a drag because it was just so hard to get around. Where we live in MD is quite suburban - no shopping centers within 2 miles, but it's so peaceful. However, we can drive 20-25 minutes and be in Bethesda, DC, Tysons Corner, etc. We have probably 30-40 restaurants within a five mile radius of our house too. Schools are comparable (if that matters). Housing prices in MD (according to Washingtonian Magazine) are rising faster in MD than in VA.
For a single person who is not terribly concerned about driving, I'd probably go to Arlington, VA, which is very trendy right for singles. However, Arlington is just outside the District and so driving around there can be nightmarish. Parking is tough there too.