I suggested Class 6 as one is better of with Class 6 card for smartphones which would be able to do 1080p recording if needed - though OPs phone does not support more than 720p video recording. Class 10 is recommended when generally requirement is of capturing RAW images or task involving requirements of write speeds of more than 10MB/s. Please correct me if my understanding is wrong.
What if still OP had opted for a class 10? Any hard or simply waste of money? Just asking coz lack of mmc class info knowledge and its compatibility with 720/1080 HD capturing relation.