Well the most important thing when choosing the mSD is the manufacture as it will dictate the condition and quality of the card itself. For example take SanDisk they are superior in this field due to research and advancement from there venture even co-op with Samsung itself. Now the class speed means at the worst case it has to meet that minimum rating which does not mean if you have class 4 card which is rated at 4MB/s that you won't get beyond that as you can get 6MB/s.
Now I been Sr Admin and I do have NSS at home so I know quality is very important. If you check few about my posts I take data integrity very seriously from corruption and other variables.
I just few minutes ago manually reconstructed the file allocation table on corrupted mSD and extracted the raw data which was 19gb and crosslinked and synced it with backup SDCard and HD, so total of 58gb raw data input which I just finished the output to 19gb and ATM in process of transferring it back to the mSD as we speak.
So go with SanDisk as that is best for general consumers like you unless you can manully reconstruct data structure and compile the raw data to useful render files. Funny part is the mSD that I use was not even corrupted as I was flashing new v0.3 kernel the permisson base strcuture was forcuflly changed to -ro thus rendering it useless and unable to be accessed via console or from device /mnt/sdcard or could be accessed from the PC as USB Drive due to its not being allocated as it was blank to begin with. I didn't want to take the mSD and use it via SDCard reader as its waste when card went blank. Since I know alot about physical data structure and layer payloads I went in forensic mode. The point was if this was POS mSD you could have kissed the data goodbye lol.
PS: I didn't give up the kernel just yet as I will reattempt it now that my data is secured. Already so many developers left as we lost 2 major dev's in the scene and I have to redownload Android SDK as I got mad 2nights ago and uninstalled it. This kernel is from hell itself :O