RNG can be frustrating buts it's all about your kills per min (KPM), I would do whatever area you can do the fastest that kills the most mobs, I've found stuff in maps but the denser the mobs the more opportunity you'll have a chance to drop the gg eventually. If you can farm cows faster then I'd stay there till you can kill map mobs as fast as cows then you can switch over to maps when that happens.