Reasons for 3DS
Cheaper (both system and games)
You get access to not only 3DS games, but DS games
Deeper 1st and 2nd party Nintendo catalogue
Excels in niche genres (JRPGs, puzzle games)
3DS is more durable
Reasons for Switch
Better tech, meaning games look and run better
Has an incredible line-up of indie games;
Has an interesting line-up of third party titles (some with questionable quality)
Nothing on 3DS is anywhere near the quality of Super Mario Odyssey, Mario Kart 8 and Breath of the Wild (I know you say you never cared for Zelda, but BOTW is amazing)
Switch is hot right now and you may regret buying a handheld console nobody is talking or playing anymore.
Nintendo's first and second party studios are going to crank out Switch games because they no longer have to spread between a handheld and traditional console.
Switch is hot and as a result, a lot of great games go on sale.
I personally feel like a 3DS is a bad investment, but that is because I have not played mine since I got my Switch.