Are you trying to compare between 2.5" vs 3.5" drives, or are you looking for "standard" or representative spec of each size? I think you should include the number of platters/platter density, since number of platters give an easy indication of general performance, power consumption and value-per-money. Too bad WD never clearly states this, and it's not easy to find out what hard drive they're using in those all-in-one drives sold by the manufacturers.
The metric that most should be interested in are the maximum and minimum data transfer rates. This will not necessarily corespond to the rotation speed which is commonly assumed. This number can usually be found only on the manufacturer's spec sheet. It is important, because the best data transfer rates are at the faster outer cylinders of the drive, and that is where the OS will be loaded.
You might also want to know the maximum cache to interface rates. With 6gb sata on the way, can the drive utilize it?
@student_sol, I'm actually trying to compare all hard disks in general. Come to think of it though, there should be two categories, "internal hard disks" and "external hard disks", right? Or should I just use "none specified" when building the database?
@cjl, would it be better to try getting the number of platters for each hard disk? Then the visitor could just extrapolate capacity/platter by division, right?
@geofelt, this is usually listed on spec pages as "max data transfer rates" or something similar? And yes, thanks for pointing out interface rates.
Capacity per platter is a better metric - if you had that, it would quickly tell you the (rough) sequential performance. Two drives of identical spin speed and platter capacity would have very nearly identical sequential performance, regardless of total capacity.