Depends on where you're often going to / commuting as well.
Vienna has a pretty significant cut in the middle due to the O&D trail - so where you need to get to affects which side you would want to be on. See map below on how once you leave 123 you can't get from one side to another w/o going way out of your way.
Also, northern Vienna will get you much closer to Tysons, and you can avoid 123 through town which can suck at times.
If you go to the southern end / Oakton, you have better access to 66 (and thus 495) from Nutley or 123. Also you'd be near the Vienna metro.
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