Whavever you get DON'T buy SanDisk... they are great cards but have issues with Galaxy S4. I bought one in the same day I got the phone, worked fine 3 days now it keeps giving me a message "SD card unxpectedly removed" every few hours. .. I tried formatting it with all kind of softwares, resetting my phone to factory settings nothing worked.... i then checked the forums and it seems that many people have problem SanDisk Ultra 64GB Class 10.
Today I'll order a Samsung SD Card 64GB Class 10. I saw people saying that after switching to Samsung ones, the issue stopped. I guess it's the safest way, going Samsung + Samsung...