Okay, I've actually just finished reading up on SD cards in another tab, so it's funny that this question was here. I believe that the 3DS(XL) only supports SD/SDHC cards, and those only go up to 32GB. So, if you wanted the most amount of space for your 3DS XL (and if you're going completely digital, then yeah, you do), you'd be looking for a 32GB SDHC card. I do not believe that an SDXC card will work on the 3DS, but I may be wrong on that. I do know that an SDHC card WILL work, so I'd go for that.
EDIT: Using an SDXC card will NOT work. Use a 32GB SDHC card.
First result on Amazon and my choice for SD cards:
http://www.nintendo.com/consumer/systems/3ds/en_na/gs_first_time.jsp?menu=using%20sd%20cards&submenu=ctr-gs-sd-compatibility --- I don't mean to **** them literally but if you do use a condom and make sure that all parties are consenting adults-Arleas