Speed vs Reliability - Hard Drives

I have a Dimension 8100, P4 1.3 GHz with 640MB RDRAM running XP Pro and only a 20-GB hard drive (bursting at the seams!).

I'm in a dilemma about which replacement hard drive to buy!!

It's between the:-

Maxtor DiamondMax Plus 9 120-BG, 7200  ATA-133 with 8MB cache (6Y120P0). and the

Western Digital Caviar WD120JB Special Edition 120-GB, 7200  ATA-100 with 8MB cache.

I know the specs sound very similar but on paper the WD drive would be slower (ATA-100 as opposed to ATA-133) than the Maxtor drive. I realise that the speed would be dictated largely by the capability of my system to process the data, which on my machine is limited to 100MHz. This aside I have also read that the use of correctly specified ribbon cables (80 strand, 40 pin) and certain system configuration settings would improve the performance?

My hesitance in buying the Maxtor is based on a number of messages I'v read in the "hard drive" community forum (by what appears to be very experienced users) which seems to indicate that the Maxtor doesn't have the reliability of the WD drives.

As my machine is really starting to slow down the "speed" aspect of the Maxtor appeals to me, but if that comes with a lack of reliability then it's not worth having.

I intend to use the new hard drive as the master and use my current drive as a slave data back-up storage device.

What sort of margines of difference in speed would we be talking about?? (Heavy applications - Excel, Access, Photo Shop and games use).

I'd very much appreciate any comments or suggestions in order to push me off the fence into buying a much needed drive!!!