I don't really see why anyone who actually knew all of their options would choose an REI Dash 2 over a TarpTent Double Rainbow.
The Double Rainbow is significantly cheaper, lighter, has more square footage and a higher interior height, has more protected vestibule space, and is constructed with more durable materials (30D vs. 15D nylon). The Double Rainbow is also sewn with care in Seattle, Washington, which is icing on the cake. It's a product you can feel decently about buying, and Henry Shires is a really nice guy who will answer any and all questions you might have about your tent.
Sure the Dash 2 is technically a double wall tent, but I think the difference in potential condensation issues between it and something like a single wall Double Rainbow are a little overblown.
Also, REI says the Dash 2 is "semi-freestanding" which is sort of like saying somebody is "semi-pregnant." A tent can either stand without the support of guy lines or it can't. I don't see a lot of gray area here.
I'm not trying to hate on the Dash 2. It seems like a step in the right direction for REI, I just think that if the Double Rainbow and the Dash 2 were side by side on REI's showroom floor, very few people would choose the Dash 2 over the Double Rainbow. Just my humble opinion though.