Ballston has slightly less of an upscale hipster vibe and is a touch more gritty. I don't have the numbers but the demographics probably somewhat reflect that. There's some less upscale apartment complexes in that area especially when you get further away from the amenities. And it really is gradual, the difference is subtle. There's still plenty of dudes in shorts and loafers in Ballston.

But if you can save significantly by living there rather than in Clarendon, I'd do it in a heartbeat. It's still nice enough and convenient enough.