The way I see it, Witches are more powerful.
Now hear me out. Both are capable of learning the same spells. But as far as fiction goes, a Witch will typically have a larger pool of options and knowledge than a Sorceress.
TYPICALLY is the keyword. A Sorceress will usually have lots of Offensive/Attack Style Magic. A Witch on the other hand will have these, IN ADDITION TO Utility, Healing and Defensive spells. (Invisibility, Regeneration, Incorporeal, Time Manipulation, etc.)
Sorceress = Fireballs, Lightning Bolts, Blasting Spells, etc.
Witch = Same as Sorceress + Utility, Healing and Defensive Spells.
Doesn’t mean a Sorceress won’t have Utility spells, but USUALLY won’t have as many as the Witch.
Witches usually have Darker Arts and Darker Magic versions of the Sorceress spells, but usually this comes at a price. (Like casting it will damage the Witch too.)
Sorceress’ are usually prettier, but there can be beautiful Witches.
In short, a Sorceress is just a bootleg Witch.