I'll say "both", just to skew the scale and be difficult!
Construct a system where casting a fifth level spell requires a 5th+ level "slot". Higher level slots can be used for lower level spells, but not vice versa. This solves the power creep.
Now, Sorcerers get their fewer spells, and cast at will. Mages can memorize a number of spells equal to their "slots" for each level... If a mage (Wizard, Bard) has four third-level spell slots, then they can memorize four different third level spells. If they have five 2nd-level slots, they can memorize five 2nd-level spells, and if nine 1rst-level slots, then nine 1rst-level spells.
Now for the different part... Sorcerers can only cast the spells they know, using a "spell point", which is a slot of a level greater than or equal to the level of the spell they want to cast... Very little change, here!
Mages, however, can learn as many spells as they can beg, borrow, steal, buy, or find scrolls for, but can only use the ones they have memorized for the day. Still no change there, but... When casting a spell, the Mage can cast any spell memorized by expending a "spell point" of a slot of greater or equal level...
Thus, Mages have some flexibility. They need not memorize
Magic Missile twice, but if they have memorized nine different 1rst-level spells, they can cast nine
Magic Missiles, or one of each, or three of one, two of another, and four of another. If they then need another one, they can cast a second level "spell point" for one, but cannot spend three 1rst-level spells for a
Fireball, even if they have it memorized.
Hence, flexibility with no other increase in power.
The only real problem is that it makes the Sorcerer less unique and powerful, by comparison... Best to use the Wizard's spell progression for both, and also give the Sorcerer the five free Feats, as well, in order to fix that.
Does this make Fighters, etc., too weak in comparison? No more so than before, methinks, but it needs playtesting... If anyone decides to try it out, let me know how it works (or doesn't) for you!