I understand about the speed difference i was more concerned about actual usage in the phone itself and if its noticable
I doubt you will notice - I don't know of any apps that will write quicker than 4MB/s. Read times are 10MB/s+.
I think i might go for Class 4 as im only going to use it to store music
makes sense - I have a sandisk class 4 32gb for my media. All the apps go on the internal memory of the sgs2.