Every other nit-picky factor being equal (quality of DM, other players, location, timing, color of the sky, my mood, presence of apocalyptic portents of doom) I'd be *more* likely to play a game with Spontaneous casting methods than one with Prepared casting methods. I'm a big fan of *most* of the books that Gygax recommended back in the 1e DMG, but I never cared for Jack Vance, and I *strongly* dislike the Vancian memorization scheme for spell preparation.
I'd hate to have to play a Favored Soul or Spirit Shaman over a Cleric or Druid, but I'd be peachy-keen with playing a Spontaneous-casting Cleric or Druid who only had access to a very limited list of spell known, but could cast them flexibly throughout the day.
Same for the Sorcerer/Wizard thing. I kinda hate that Sorcerers lose four bonus Feats and get a slower spell acquisition rate, but if a DM said that he wanted to ditch the Wizard completely and make the current Wizard class cast Spontaneously and be limited to Spells Known like a Sorcerer, I'd be all for that.
I'd be even *MORE* for a game which had both Spontaneous and Prepared casters of all spellcasting classes (cause some people like both, the freedom of spontaniety and the tactical and strategic resource-management of Vancian). A Prepared Bard with a sheaf of spellbook pages he's cobbled together over his travels, learned from his half-forgotten noble education and not some draconic heritage ('cause not every single musician in the world is descended from dragons!). A Spontaneous Paladin who knows only a few of the spells on the Paladin list, but can cast them flexibly and a little bit more frequently.
Best of all, Clerics and Druids limited to *either* be Spontaneous casters, with a very small list of Spell Known, usable flexibly, *or* Prepared casters, with prayerbooks and holy scripts that must be maintained and developed over time, just like a Wizard's spellbook.
None of the current situation, where Clerics and Druids (my favorite two classes, btw) automatically get access to every single spell on their spell-list, while Wizards have to pay for / aquire their spells and Sorcerers have a tiny subset of their spell list as Spells Known. 'Cause that's, IMO, at least 50% of 'the problem' with Clerics and Druids. They get it all, for free.