I really admire Morrissey's ability to change his vocal persona according to what he's doing at the moment - he really does sound completely different at different phases of his career, from the understatement of Southpaw to the almost feminine eerieness of Viva Hate to the range-attack-on-falsetto-mountain assault of Ringleader.
To me this seems like an exclusively post-Smiths phenomenon, though his voice did evolve a lot over their career. I think it had to; his performance on The Smiths has a classically first-publication quality to it, a bit like Fitzgerald's This Side of Paradise: promising but technically unimpressive, exceptionally pure and beautiful, and expressive of a youthful self that disappears by the very act of being expressed. I don't believe that this is a bad thing -great creative people have got to begin to grow sooner or later- but I do believe that it's an inevitable change.
(I would never think to characterize the Smiths-voice as "childish" -I was hung up on how deep it was- but you're totally right, Wolve.)
Anyway, my favorite Morrissey is Southpaw-era, but current is an extremely close second.