Hubtime
I use a 2.2gb magicstor microdrive with my 2210.
It use an additional 12-16% of power in comparison with my SD card (sorry don't have an additional CF memory card to test).
This is using Betaplayer - screen off and microdrive option set to play music (ogg files - additional usage between 12-15%) and movies - microdrive option set (13-16%). I have around 35mb of free memory on my 2210 and BP copies in batches to my main memory to reduce the drive spinning. This is based on about three months 'real world' commuting usage.
I assume that the usage would be similar on the 4700 (unless you had movies encoded at a higher bitrate due to the screen, which would require more disk access).
I haven't seen any 'scientific' testing of a comparison between the two, but I find 'real world usage' a better gauge anyway
Hope this helps.
Rob