The answer depends how someone intends to use their 3DS but IMO, 128 GB formatted in FAT32 + 64 KB cluster is best overall: cheap, fast, and big enough for a wide range of casual, novice, and advance users.
256 GB is for the super hoarder where the 2DS/3DS is their only possession: ultimate retro video gaming rig. I list 256 GB cards on my signature in case someone intends to later migrate to the Switch, their card isn't too small.
Anybody here have a Xbox series S and a series X, is the performance difference noticeable? Not worried about 4k. I have a series X but not a series S and was curious. (Reason- I was thinking of buying a series S for a second tv, for when grandkids come over, to keep them off my X)