In P4, you have like 5x the amount of free days that you do in P3 and all the Slinks advance a lot faster(Especially given that there are so many that advance just by visiting them once and one that doesn't take any time) . It's a LOT easier to max them in P4.
As for Persona use in P4...in my game, it felt like each boss had a particular Persona that you were "supposed" to use for the boss(Like, Sun Yatagarasu has, by itself, the entire set of skills you need to completely shut down Mitsuo, and he's exactly the level you should be to fight him), so the Persona progression felt a little TOO static. Also, since ultimate spells can't be transferred in P4, there is one Persona that is the "best" for each elemental spell type, and that's that. In P3, you could pass Ragnarok to anyone with Fire affinty, and could make any of them into an awesome fire Persona. So the system felt a little more open and fun to play with, while in P4 you just got Yoshitsune and stuck with him.