Honestly if you just get rid of all the ways prepared casters can cast spontaenously (including leaving open slots to prepare later, Uncanny Forethought Feat, etc), and drop the spell level acquisition on spontaneous ones down to match the prepared ones (I have never understood the reasoning behind that), I think they'd be fairly equal. The thing that always made me never want to play one was the delayed spell acquisition, and the fact that you could slap spontaneous casting on a wizard with a single feat.
Another thing that could help out would be to give the spontaneous guys a way to cast the oddball stuff as a ritual or something (or drop the XP cost from Limited Wish and Wish so they could use those for that). You know, like Contact Other Plane, Contingency, etc? The things that wizards will learn because "why not?" and sorcerers will never want to take up a known spell slot with. Of course, they can already do this via Spellstaves, Knowstones, etc, but if it was a class feature it would be more encouraging to newbies.