Primarily Fluff, Metamagic, ritual casting and number of spells known.
Giving +Proficiency to number of sorcery points would help.
I've also supported a more limited version of Spell Versatility, such as having it be something you have to invest in before you can do it. E.g.:
"Starting at 4th level, each time you take a long rest when you have at least one sorcery point remaining, you can bank one point into a Versatility Pool, up to a maximum of your Sorcerer level. You may spend points from this pool when you finish a long rest to change one of your spells known equal to twice the level of the new spell or the old spell, whichever is greater. All points in your Versatility Pool are lost when you gain a new level." Much more limited than just once a day, and changing powerful spells is a
serious effort that requires a large investment--something you can't simply DO on a regular basis, and in fact you're very unlikely to ever change more than one or two 7th level spells ever, as that would require
two weeks of long rests.
Alternatively, I would have liked to keep SV the way it was, but offer the ability for Wizards to build up
(semi-)passive spell research that would let them permanently learn a new spell after JUST spending downtime (no gold, just downtime), in a specifically "your random personal research has finally resulted in a breakthrough, pick the spell you want" kind of way. Thus, the Wizard really DOES fulfill the fantasy of actually spending lots of time on personal research which...uh...is kind of not present in the Wizard we have now. The current Wizard just copies spells from other people or spontaneously generates spells with no time actually spent
studying. Wizards would get a small, nice thing that is super flavorful and class-fantasy furthering, while Sorcerers would suck less.
As it is, we have Sorcerers who are more (as others have said) like mutants stuck with potentially crap powers, and Wizards that are a lot more like virtuoso composers suddenly belting out inspired pieces (new spells on level up) rather than meticulous researchers slowly building a body of work.