The 3DS Mario Golf looked good, was as fun as it always is, and had a lot of courses (especially with the DLC).
But I definitely preferred playing the GBA one with the great RPG mode. Since with was Camelot and used their Golden Sun engine, it felt and sounded like I was playing a new Golden Sun game, just with golfing instead of battling. (I played it a long time after it came out so it felt really good to play something in the Golden Sun engine)
Those are the only two Nintendo Sport games I've played, so I always look for them to have a deep single player before buying them (Mario Gold was part of that Humble bundle I think, I wouldn't have gotten it otherwise). I'd probably just end up buying Mario Tennis GBA or the GBC games instead of getting any modern Mario Sport games if they keep leaving out meaty single player content.
That Golf RPG that's coming out on the Switch looks pretty solid. Another case of indie devs filling the void Nintendo leaves when they don't make stuff a chunk of their audience wants (Axiom Verge with the lack of Metroid, an ever increasing amount of futuristic racers with no F-Zero, etc.)