There's a lot of variables. Certain places in zOMG give higher amounts of gold, there's loot boosters such as a gold potion or Fortune's favor ring, amount of decent booty-grabbing threads/tanks, gold nerfing, and so on. Do a little experiment and see which you prefer doing. Personally I find myself making more gold with BootyGrab than with zOMG. But zOMG can be more fun and worth the time spent.
Like what Kekora said - it's really a thing of personal preference.
I find myself making lots of gold on Booty-grab AND zOMG but I think zOMG is a much more fun way of making gold. Plus with zOMG you can really get into it and go crazy and not realize how much gold you're making because it's so much fun!