Wizards = versatility over the long term
Sorcerors = versatility over the short term
In other words, over the course of a given day (or combat), a Sorceror has more options than a wizard will, not being bound by the spells he has prepared in the morning. A Wizard, though, over the course of a week, will have more options than the sorceror, not being bound by the spells he has chosen to learn for life.
In practice, Sorcerors are better combat mages (more spells per day = more fireballs per day!), and Wizards are better dungeon-crawling mages (being able to pull some rarely-used, specialized spells from their book). It's sort of the difference between Fighters and Rogues (especially with the Sorceror's weapon proficiencies

)
Personally, I prefer Sorcerors, but only because I hate bookkeeping. Just a little X next to the appropriate level spell. Keep them stocked with wands, and you're set.
Wizards are however pretty universally accepted as the "better" class (their spell progression pretty much nullifies any argument otherwise, IMO... although I think the margin between them is smaller than most people think.) It all depends on the type of gaming you prefer: If you're a strategic, meticulous thinker, Wizards are a better fit, and if you're more of a heroically-kick-in-the-door type, Sorcerors are more appropriate.