Does facility complexity factor in as well?
Yes, facility complexity does factor in. A lot of different variables go into calculating facility complexity. From what I heard, things like:
Airspace volume
Number of primary and secondary airports
Number of instrument approaches
Military traffic volume
It all gets taken into account when determining a facility level.
It's not such an easy question to ask whether or not EnRoute vs. TRACON is harder. Since I am not a controller, I won't tell you what I think. But from what I have heard, most
major TRACONS are thought to be harder then most Centers. But within a Center itself, you can usually find a specific sector or two that are f-ing crazy hard. Usually low alt sectors.