I use a 64GB SanDisk SDXC, reformatted to FAT32. It's a beast.
Also yeah, be careful of fakes! I mostly buy my stuff from Amazon and I usually look for sellers that I can get Prime shipping with, because Amazon is really big about making sure Prime members don't get ripped off.
Dunno if anyone's ever done it with SD cards, but I remember a while back there was a rash of people selling fake, high capacity USB drives. They'd show up as the right capacity in Windows but they were set up as a loop, so e.g. you'd have a "128GB" stick but it's really 1GB, and when you get to the end of 1GB it just starts overwriting the stuff at the beginning of the stick.
Also! MicroSD cards will work with a MicroSD to SD converter thing (which most of them come with) *but*, from my experience, those converters don't last very long, so you're probably better off just getting a full size SD card, especially if you only plan to use it with the WiiU.
Basically, because I'm bored and feel like being verbose and trying to explain shit, there's three types of SD cards:
SD
SDHC
and SDXC
SD was only up to I think 4GB (correct me if I'm wrong) and used a filesystem called FAT16. SDHC was up to 32GB and used FAT32. SDXC I forget the max on it but we haven't hit it yet afaik, but by default it uses exFAT which is a filesystem that was only added to Windows in Vista, and is patented. Hardware wise, there isn't a whole lot of different between SD and SDHC, and there's *no* difference between SDHC and SDXC. So, most manufacturers that are cheap (and Nintendo is one of them) just build in SDHC support, because the filesystem used in that is free and open. But the thing is, even though Windows and the SD people don't want you using FAT32 on SDXC cards, it actually works just fine. It's slightly less efficient as far as space due to increasing cluster sizes (something you probably don't really care about). Basically *any* SDXC card (>32GB) should work as long as you use a utility to format it Fat32.
http://www.ridgecrop.demon.co.uk/index.htm?guiformat.htm Here's one that I know works just fine for that purpose. Just make sure you're formatting the right drive.Z
And if all that just made your head hurt, just get a 32GB card, it'll work fine out of box.