My info is about three years old, as in pre-covid, but I remember that Flamingo has/had two pools, one family-friendly and the other more or less reserved for adults. Bally"s, if memory serves, has one large pool.
Another consideration is whether either pool is open to kids on the weekends. And of course, since different casino megablobs own the two respective properties, you'll be able to get in one for free but not the other.
(Remember when pool access was an amenity you'd take for granted as part of being a guest?)
I'd call each property and ask specific questions; I've learned to not rely on whatever is on their websites.