The X08D is the newer frame.
It uses a newer and improved screen panel from a different manufacturer. It keeps the same resolution as the X08C but it has a better backlight, giving a better luminance to your pictures.
The Software and Interface are the same on both models.
NOTE: The X08C/X08D does not have an Internal Memory. It is a Plug and Play frame that simply operates by you plugging in your USB Drive or SD/SDHC Card (with pictures already loaded onto them) into the side of the frame and viewing photos in this manner.