Formatting is better for the card. Deleting on the pc/Mac can cause issues down the road. Copy everything off to a hard drive, format the card using mavic.
A lot of the issues described in this forum about problems deleting files can be avoided by simply "ejecting" your disk in OSX (on Windows you click "safely remove hardware") before you pull out the SD card or adapter.
No one ever uses these but this is the major cause of things not being properly moved/copied/deleted/recycled etc