Been a while since I drove there at rush time, but I know the biggest backup of 270 is out at the Frederick end anyway where there aren't enough lanes; you don't get to avoid that going to Shady Grove. This is part of why it seems like such a toss up to me once you get to that point. If you go on 355 from Shady Grove you're already dealing with much of the crap that you're trying to avoid on the Metro, particularly on your way back out if you've parked at Rockville or White Flint. If Shady Grove is full I think I'd just drive all the way in, even though that sucks too.