My opinion is probably skewed, I only do rtb and no direct buys, so my categorical targeting is somewhat different than what OP is doing. BT generally does better for me because 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.)

I'd usually rather get 100k of those really expensive impressions, than 1m of less targeted (cat) impressions.

Advertisers are trending towards behavioral, it is a lot more powerful and the available inventory is much much bigger. Rather than limited to only sites about bacon, the author get buy ads on any site and (in theory) get the same performance while spending less.