I used a microSD in a New 3DS XL because it has a microSD slot and a normal SD in an original 3DS XL given it's SD slot:
Prices on both seem to float between $16-$18 on a regular basis.
I just put a 128GB SDXC SD card in an original 3DS XL and it works fine (FAT32 64K block size). I realize the original 3DS have SD and not microSD slots but wouldn't an microSD SDXC in a SD/microSD adapter work fine? (I have a bunch of adapters given how so many micro SD cards come with them and I never need them)