As an apple tech for many (10+) years I would like to point some facts:
all apple OSes from 10.0 till 10.4.11 were more faster and more responsive in every newer update, 10.4.11 is the fastest OSX that currently exists.
The introduction of 10.5 was the worst product apple ever brought to market, because of MANY bugs and it felt like an elephant rather than a leopard.
with every update on the 10.5 it became better all the time but it was realy useful after 10.5.2 on intels and after 10.5.3 on PPC machines.
Currently on 10.5.6 the OS is ALMOST as stable as 10.4.11 but not as fast.
I belive the nature of Universal binaries of the OS will never let it be as fast as 10.4.x. 10.4.x came in 2 distributions and updates, different for intel and PPCs.
But feature wise (and Future wise) 10.5 is in advantage, most of newer apps requires it, and some features like spaces bring the OS to another usability and friendliness level. For slow machines <=800Mhz without core image graphics supported 10.4 is the best OS. for all other machines better get 10.5 that also in some actions is faster than 10.4 because of the use of core image graphics cards.
As it comes for hard drives......
both mac and PCs use the same hardware..
but till early G4s, apple used a custom firmware that it had enabled an option in every hard drives firmware about reliability, all HDs that was sold on the PC market had that option disabled. AFAIR that option made hard disk slower but more reliable.
