http://www.amazon.com/SanDisk-Ultra-Class-Memory-SDSDU-032G-AFFP/dp/B007M51J3I/ref=sr_1_8?ie=UTF8&qid=1368060210&sr=8-8&keywords=32g+class+10+sd+card
http://www.amazon.com/gp/product/B004Q3C98S/ref=sr_1_3?ie=UTF8&keywords=sd card 32 gb&qid=1366337839&sr=8-3
Are you planning to replace the SD card that the 3DS comes with or something? o.O
Well of course I'm replacing it...
I say the SanDisk Ultra ^-^ it's water and shock proof, which seems very helpful.
Also, The SanDisk Extreme seems more for HD video cameras, and you just need an SD card for your 3DS.
Edit: Ah, both are water/shock proof, I'm sorry about that >.< The Ultra is cheaper though.
I don't think it matters much since they're at roughly the same read speed, and likely write speed as well. Both are also supposed to be water/temp/shockproof. Save a few bucks, or spend a few more for slightly faster, maybe even negligible, speeds. I can't say whether or not the increased speed would be noticeable.
That reminds me, I've been wanted to replace mine with a 128gb sd when I found out it apparently does support that high. $100-150 and I'll probably never have to upgrade the SD card again! lol
Spoiler: Nearly 1 million blocks show (http://i.imgur.com/GltZD.jpg)