they're both very easy, but i'd say in my experience guppies are easier, make sure to have plenty of room in another tank for the many babies. also, guppies have a lot more fry, at least mine have. i get 30-40 guppies, and 15-20 mollies. but literally a few males and females in a tank is the extent of the preparation lol, they're breed with no coercion. but make sure not to put just one or two females if you use several males, they will chase and stress them out, always more females than males. i usually wait three weeks and then move the female to a new tank for a week to let her give birth.