I'm very new to the publisher side of media buying - my feeling after reading OP's post was this was the case. We currently run AdX on our highly categorically sorted site (with no smarts to it) - are you saying then it would be better ROI for us to build out ad serving based on behaviour rather than category? (Category would be much easier for us, I imagine this is just a case of sending the category to Doubleclick.)
I have no publisher side experience, and I only do rtb - no direct buys. Traditionally BT generally does better for me. Data quality tends to be better (you can get really specific, like a user with HHI 100k+, owns a Ford, AND is in market for a new minivan).