Both are fantastic, I'm running tiger (10.4) on my laptop.
Panther (10.3) was a big improvement over Jaguar (10.2) but the upgrade to Tiger has been less marked.
The whole point of Tiger is that its fully 64bit functional so the Powermac G5s should really fly with it. For us operating it on older systems the improvement is less obvious.
Spotlight is nice to have, very clever. It searches even within PDF files. So if you are looking for something lost in your hard drive its good.
IMHO Dashboard is not worth 50pence. You need to be online on broadband the whole time for it to work. I've got a weather widget on and thats it.
Overall I'd say the upgrade from Jaguar to Panther is definately worth the money.
Panther to Tiger....hmm jury is still out.
Have you tried
Macformat forum lots of advice over there.
