Faster than ram? not likely. Probably faster than the current FLASH-ROM though, it's pretty slow compared to your average hard drive or even a compact flash card.
Now i'm no hardware guru, but i'm guessing that a hdd that small, will no be as fast as his bigger brothers.
Unless you ARE a hardware guru on these kinds of devices,(ant that's realy not meant sarcasticly) perhaps it's a bit more efficient to simply pull up a few white papers about the speeds of those 2 types of devices.
Where the most crucial point will probebly be the initial time it takes to find a random point of data without prior spin up. (of the hdd)
becouse pda's put most of there stuff in memory the hdd will probebly only be spinning when it's actualy needed. To preserve energy.
Which is another point of interest, how much energy is a hdd going to take.
Now i don't know the answers to those questions, but i'm guessing the flash would win.
simply becouse it's designed from the beginning as a energy preserving data storage, and a hdd isn't. (although the mini hdd's offcourse are designed with this, but i'm more talking about the global technologie involved)