The Class-6 card will be 3 times faster than the Class-2 card (6MB/s as opposed to 2MB/s). Transfer speeds are the only difference between them.
A Class-2 card is perfectly sufficient for playing Mp3 music files, the only time you would even notice the higher speed of a Class-6 card is when transferring large amounts of data onto or off from your phone. Unless you plan on storing large files on your phone, there is really no need for a high-speed card. They are designed primarily for use in digital cameras where being able to write large image files to the card quickly can be a very useful feature.
Clark