Low gear, vork > zulrah. Middle gear, Zulrah > vork. High end (dhcb etc), vork > zulrah. However, they're pretty close all the way. So close that the boss you can do most consistantly will get you more. I personally swap.
I do agree that vorkath demands less actions than Zulrah. However, especially once you get into it, Vorkath will demand far more attention than Zulrah. Especially if you get tbow, then zulrah will be basically semi-afk, and supereasy to multiacc.