Well, if you already have a 3DS (or 2DS), I wouldn't see why you'd want to buy a more or less duplicate of the same system simply for a few quality of life upgrades, maybe some better graphics. As of this topic there only seemed to be one exclusive game, and I don't think they made much more since then. Hyrule Warriors, maybe, but that's about it.
The Wii U was quite fun despite its commercial failure and while I never played Happy Home Designer, the other two games mentioned seem to be far more popular, so I'd say Smash Bros and Splatoon are objectively the better games. Especially when you're getting two games versus one game and a slightly upgraded console.
Does anyone even remember Happy Home Designer anymore, anyway? Seems like one of the many games Nintendo marketed for a little while then forgot about. Especially since it was near the end of the 3DS' lifespan, they made a lot of games like those back then... Pikmin, Chibi Robo, a few others.