Sorta both, but not quite? Swap!Papyrus is much better at creating puzzles than Sans since he actually studied it; he’s also way more interested in puzzles — it’s something he’s actually passionate about. And Sans could use the time to clean or cook or train or do anything else instead, and that would be more productive.
So, really, it’s killing two birds with one stone.
He’ll notice in the retrospective, but, like I’ve said above, he gets to do something that makes him happy, so he wouldn’t really mind.
Actually, I’m pretty sure that even the original info for Underswap never said that they dropped those quirks — it only said that Swap!Sans enjoyed riddles and Papyrus pulled pranks. So I see zero reasons to take their original quirks away!
I went to a catholic primary school that taught children this concept called ‘JOY’: meaning ‘Jesus first, Others next, Yourself last.’
9 years later, here I am with diagnosed anxiety because I have had no idea how to take care of myself mentally and I was keeping a constant state of anxiety on worrying about people who never worried about me.
Moral: learn to look after yourself before looking after others.