It's the maximum burn speed.
Here are the relevant specs for CDs burning with the SuperDrive on a current iMac:
* Writes CD-R discs at up to 24x speed
* Writes CD-RW discs at up to 16x speed
So, using the above speed, either of the discs you mentioned would exceed the maximum speed that the iMac's SuperDrive can burn to them. Generally you want to get media that is equal or faster than the maximum speed of the drive it'll be burnt on.