As above, I'd rather have my own custom slann, but out of the two I'd rather have Kroaky I think. IMHO sticking a character as expensive as Lord Maz on a steg and shoving him out on his own is asking him to be cannoned, bolted or obliterated in some other equally spectacular fashion. Kroak's immovable TG anvil will always come in useful, no matter how much it costs.
Kroak is the better of the two. But with a regular slaan you're better off all you really loose is the no-limit on dice per spell and the special spell, you dont really loose unbreakable because LD9/10 cold blooded re-rollable is damn near the same thing.
it's only damn near the same thing if your running him in a unit of templeguard, otherwise the combat modifier will eat him.
BYO Slann as GW always make Lizardmen characters virtually unplayable since 5th. The fluff starts good (with Kroak clearly outfluffing MM) but then implementation leaves us with something you can't use.