My suggestion is getting micro SDXC in case later you happen to upgrade or gifted a N3DS, you don't have to buy new memory card.
You need Class 10 for good speed.
For 32GB or bigger card, you need gui formatter (Google it, can't link here) to format the SD to FAT32 file system for the 3DS to use it.
Stay away from SD sold on Ebay. Always heard bad thing buying from Ebay. So unless you go for absolute quality and performance in brand line Sandisk Extreme, Sandisk Extreme Plus, Sandisk Extreme Pro, Samsung Pro, Samsung Pro+, there is not much performance improvement between other lines of Samsung and Sandisk. With other brands, rule is stay away from Kingston (I had no actual experience with them but many Tempers warned me so I passed the info along) and pick anything that suits your budget.
For price preference: you're late to the party. Around Thanksgiving, there were some absolute killer deal like Samsung Pro+ 64GB sold at Fry's for $19.99 before tax
http://www.amazon.com/Samsung-Plus-MicroSDXC-Memory-Write/dp/B012DTJ8NU/ref=sr_1_1?ie=UTF8&qid=1451964783&sr=8-1&keywords=samsung+pro+ . BestBuy and Samsung.com sold it for $29.99 before tax and Shipping.
Then Amazon had Sandisk Extreme Plus for $29.99
http://www.amazon.com/SanDisk-Extre...1964837&sr=8-11&keywords=sandisk+extreme+sdxc
These are the best micro 64GB SDXC in term of performance (can be used for Ultra HD video camera like Go Pro). And those price are cheapest ever seen in retailer stores without coupon.
Edit: This thread should go to 3DS Hardware section.