My answer is exactly about the practical difference in short. Their ancestry doesn't come as just "in-lore justification", it has effect on how and what they cast mechanics-wise.
Ok, fair enough. Then why don't you answer his question listing these practical differences with something that is not "One had to learn magic and the other was born able to do it", which is frankly mechanically irrelevant.
Admittedly I'm not exactly up to date with the nuances introduced in 5th edition, but as far as I know the main difference between mage and sorcerer (aside for relying respectively on INT or CHA as main stats) has always been that the former has access to way more spells and more powerful ones in relation to the current level, while the latter can use more spell slots per-day before resting.
