"the Cleric's later game light/dark ability" *is* "the super-powerful later-game magic capacity that everybody tells me about"!!! The only magic in common for both Druid & Sorceror is elmnental, & it doesn't come close to the mirrored paths (Light/Dark).To really thrive in late game, at least two Light/Dark characters are a must. One should obviously be a Sorceror, for Fly/Town Portal/Lloyds Beacon. Many people like two Sorcerors, but then you need other healing & if you don't want the Cleric, you're either tripling up on elemental with a Druid, or going with a secondary healer with no backup (Paladin). XPSS probably works if you already know the game well, but for your first full playthrough you need healing, you need elemental, you may as well at least go with the ones that also give you Light/Dark into the bargain.
My personal playstyle always favoured might over magic (at some point you get sick of continually entering turn-based, & you're tough enough to just run up & bash away at many things, & drop into turn based if you need to heal up or TP out), so I always favoured KKCS. This also gives you plenty of protection in the early game & allows plenty of room to quickly develop those useful secondary skills like Disarm & Repair.
But I'm not so worried these days about clearing every map - if you want to fly around & slaughter every Gold Dragon in the game, or if you prefer a magic-heavy playstyle, I'd probably go with XCSS with only one character continuing to develop Light for the multi-buffs. Heaps of Light/Dark, pure healer, the ability to share the elemental schools between the Sorcerors, & two sources of Fly/TP/LB. Peter2 favours ACSS but I don't find a use for that much elemental. At this point the "X" in XCSS really depends on personal preference & playing style
