I commented mostly on the Westin Tysons Corner, but mention the Sheraton in comparison in
this post.
Personally, I would just stay at the one that has the better rate for the nights you are looking for. Staff seems more attentive at the Westin, but it has fewer services, no gift shop or drink vending machines anywhere (inconvenient!), and as you will read in my previous post, didn't have an ice machine on my floor, so it's not quite "full service" in my book.
Location-wise they are comparable if you have a car; if you don't, the Sheraton has more "action" going on around it, but nothing you'd really want to walk to, though you could if you wanted to, since it is located off a walkable service road. The Westin is located along busy Leesburg Pike and not practical to walk to anything.
To me, both properties are just "meh" with upsides and downsides to each that make them pretty equivalent.
Sheraton has a lounge that I did not bother to visit. I don't believe the Westin has one. If they do, I can't imagine where they put it, and there is certainly no indication of one in the elevator.