While the transfer rate indicates absolute maximums, I'm not so sure they represent typical, real-world transfer rates. *If* the USB drive is able to use 90% of it's available bandwidth, while the conventional laptop hard drive is only able to use 10%, the USB drive will be much quicker. Either way, USB drives should have much less latency as hard drive spin-ups should take much more time than whatever form of electronic scanning a flash drive uses. However, with the increase in HD cache, perhaps the mobile drives can offer comparable latency.
This article tests some mobile hard disks.
Unfortunately, they don't compare them to USB flash memory. The disks they review have access times that are measured at 14-18 ms, even though they are all listed around 12ms. Transfer rates are in the 20-40 MB/s range, and they tend to have 300-400 IO/s.
Dell's site lists the cheap HD (which I would probably get) as being SATA interface, 5400 RPM, 12 ms access time, 2 MB buffer, and a 5 second spindle starts time. So, because of the smaller buffer, it will probably be a little slower than the hard disks reviewed in the article, and will not come close to maxing out the SATA bandwidth.
Unfortunately, I have yet to find people doing this kind of testing on USB flash drives. I think I will test the flash drives I have, and then post the data.