Regarding "he heals for more" - that is largely down to what equipment you put on each of them. I'm not entirely certain, but I believe that Vanille probably maxes out better for cures if she has a fully upgraded Healer's Staff (or whatever).

However I've found that as you go up the levels, you can pretty much improvise with all of the characters. I've had a very unlikely trio come out victorious against Neochu (with his Picochu minions) recently despite forgetting to adjust far from the main three paradigms - I had no RAV or COM in my paradigm set at all! I won using SEN/MED/SAB, with Vanille casting Death and Snow using Vendetta.