As far as Cleric/Wizard vs. Cleric/Sorcerer is concerned, I say Wizard if you want to gain access to 9th-level Wizard spells. As it stands, you won't be able to gain access to 9th-level spells without going into Epic levels. But if it's 9th-level Cleric spells that you want, then go ahead and take Sorcerer. It'll make things easier, to, since now all you'll need are good Wis and Cha stats, rather than good Int, Wis, and Cha stats (Cha for the Turn Undead).
Also, has anyone considered Druid/Wizard for a Mystic Theurge? I'll have to think about that possibility, later.
Though what I DO like is the thought of playing a Cleric/Wizard/Mystic Theurge in the Forgotten Realms who's a worshipper of Mystra. Now THAT'D be cool, and completely appropriate for Mystra's clergy.