I would suspect the diamond's built in memory would be somewhat faster. (Just based on the electrical interface)
My reasoning:
If I remember my design specifications, SD specifications call for a 4 bit interface.
The built in memory on the diamond is very much more likely to be a 16 bit interface (or 32) to the actual chips themselves.
Now, SD has come along way with the speed, and the actual devices "aren't THAT fast" compared to a regular PC.
Will it be a noticeable difference? Probably not. For the hardcore speed freaks, could be a few ms. in access time.
