For those of us who have modern Digital Cameras or MP3 Players capable of using Type-II CF Memory, there is also an option known as a Microdrive.  My observations of recent pricing have both running competitively for a given amount of storage.  So, how do they compare?

First, let's clarify a point about CF Cards - Type I vs. Type II.  The difference here is in the thickness of the card - about 3.5mm for Type I, compared to about 5mm for a Type II.  The electrical connection, a 50-pin socket, is the same for both types.  So, a camera which has the newer wider slot to accept Type II cards, will also accept Type I cards - and indeed, the majority of cards on the market are made to the Type I thickness, so they will fit anything ever made to take a CF card.

A Microdrive is a miniature Hard Drive, usually using a 1 inch diameter disk.  Its housing is sized to the thicker CF Type II specs - so some, mainly older, devices which have the narrower Type I slot, won't physically accept a Microdrive.  And not all devices that accept a Type II card will support a Microdrive, through many newer devices do. 

Another thing to look at, is the Speed at which a Card and Microdrive you are comparing handle Data Transfer.  Read my Guide titled Understanding The Speed Ratings Of CF/SD Memory Cards to learn what these speed ratings mean, and how they relate to your device's capabilities. 

For a quick example from that Guide, my Sony Alpha 100 DSLR will handle a Data Transfer Rate of 9MB/second, which is nicely matched to the now common 66x speed CF Cards (such as Sandisk's Ultra-II line).  The Hitachi Microdrive which I have is rated at 7MB/second - which would cause some buffer clearing delays for a real power shooter, but is no problem in my normal usage.  Where I actually SEE the speed difference, is when I am transferring my pictures to the computer.  7MB/second from the Microdrive vs. a Read Speed of 10MB/second from the Card, IS a noticeable time difference when you're moving a few hundred pictures around.  Manufacturers are coming out with new stuff all the time, so you may well find a faster Microdrive than this example, just as you can also find faster Cards.

Now, consider the basic physical difference between the two.  The CF Card is a Solid State storage device - no moving parts.  Barring a major electrical zapping, or gross physical abuse, a CF Card will be a reliable storage medium, probably outlasting your camera or other device.  (I have a few Cards in my collection that are on their third camera, and still going strong.)  The Microdrive, on the other hand, has a number of small moving parts inside - with the potential for a jolt or vibration to cause major and permanent damage, just as it can with the hard drive in your computer.

A final point - Power Consumption.  It is rare that I encounter a Digital Camera owner who is satisfied with the life of their Battery, so consider this fact:  CF Cards use very little power, while Microdrives draw on average 20 TIMES the juice! 

Need any more convincing?  CF Cards are the way to go, for reliable memory that will survive the real world.