Re: 5 better then 3 ?
Here's how the platters/density debate stacks up, from first principles:
Picture a drive with one platter and two cylinders. With heads on each side, this gives you the potential to read/write two tracks at once out of the four total.
Now quadruple your data by adding three more platters and you can read eight tracks at once out of the sixteen total. Consequently, your sequential read speed quadruples.
However, it take the same amount of time to seek between the two cylinders as it did before, so random performance doesn't increase much.
If instead you quadrupled the density of the drive, you'd now have one platter with four cylinders each with twice as much data per track.
So your sequential read speed doubles, but your seek time also decreases (because the cylinders are closer together, reducing head travel time), improving random performance.
In short, if you want raw sequential performance, increase the number of platters. If you want random performance, increase density. If you want to store gobs of data, increase both.