So I like to go for more evolves over larger teams. The biggest reason is when you get to the 3rd zone you have to be really careful about enemies who your whole party, or ones that seek out your weakest monsters. I usually try to have all my monsters at their 2nd form by then, or 2 monsters in their 3rd form. Naturally this partially depends on the monsters you are using, for instance Kabu, the beetle, does well fully evolved, but things like the Krog(frog), or Chikia(the orange bird) give more benefit to a larger party. Generally if I feel like I have extra really nice items, that also pushes me to use more monsters, vs if Im struggling to find things to equip I lean more into evolving the ones I already have.