This is entirely subjective per my experience:
- the 10.5 'feels' more responsive.
- the beachball is a very frequent event in the 10.6, specially Safari, sometimes even when switching tabs which is quite annoying.
- I haven't rebooted the old MBP in months. Sitting pretty on my desk. The new one has frozen few times and occasionally I feel the need to put it out of its misery myself.
Seriously, I know people who have a reason not to upgrade. You don't seem to be one of those people. Those people are on worse operating systems than you are.