it's pointless to try and compare two completely different games.
You mean, besides which one you think is more fun and/or original? razz
It's not like you need absolutely objective facts when you're sharing which one you prefer.
Personally, I prefer zOMG. More gold, definitely more original, and even less grinding is needed.
I must say, however, I've been playing Monster Galaxy far more than zOMG. Though more grinding is required to get ahead, it is, overall, a more bite-sized game than zOMG. I can pop on for an hour or 5, level up some stuff and maybe evolve a couple things, then drop off again. When I play zOMG, I usually go on fully intending to play for hours--and with school on now, I can't afford to do that.
Also, being reminded of Pokemon is pretty nice.