If it had to be a choice between the two then hard drive is the obvious choice. With a hard drive you can tweak and change your sounds and then save those changes back to the hard drive to load again another time. With a CD you obviously cannot do this because CD roms are read only. Secondly relying on the 3.5" disk to save all samples (which you would have to do if you had no HDD) is virtually unworkable if you use the ASR seriously.
