One difference between FC and SC (as well as 3rd) is that Shining Poms (the hard-to-kill-with-anything-other-than-S-crafts-and-Shadow-Spear-monsters who give insane experience and sepith) no longer appear as stand-alone monsters on the field, but randomly accompany certain monster formations in certain areas. (if you hit those formations and no shining poms accompany them, you can run away, engage them again and have another try) This is one of the ways in which you can get previously benched characters up-to-date faster. One of those monster formations is, I believe, the metal rabbits outside the southern entrance of the city.
Because of the level scaling, one of those encounters might just be enough to instantly get your newest addition on the same level as the rest. (it's always fun when a behind-the-curve character gets a 9999 experience boost)
In terms of casters, I believe their Ats-stat is usually similar when they're at the same level. One of them is just about versatility because of a flexible orbment with only one elemental-constrained slot. The other is about raw power to contrast a more restrictive orbment because most of the equipment that gives big boosts to the Ats-stat you eventually get is gender-exclusive and with it each offensive art they lay down, even the more basic ones, hits like a mini-nuke.