Different hard disk sizes, same series = equal performance?

As is the case with any hard drive, the read/write rates get lower and lower the closer to the edge of the platter the drive head gets. Given this fact, if the specs on both drives are exactly the same, except for the capacity of the drive, the performance will be the same or slightly better (because of the higher data density) on the larger drive, until you hit the higher capacity regions.

I hear it really depends on what you are doing to the HD

Seek times are faster on smaller HD

Write times are better on larger HD

If you go to the HD chars in toms hardware and compare WD rpator 74 Gb and WD raptor 150 Gb.... you can see each drive has its advantage and disadvantage depending on the taks you have given it.