Macs and PC's use essentially the same hardware now.
Because of the way the software is written for the macs some applications run faster on the mac. AND the same is true for some applications in Windows. In the end its about the experience and personal comfort. Both are subjective and therefore subject to differing opinions.

And to address the speed of the old power pc macs - my dual 1ghz mac outperforms my 3ghz work pc in most daily tasks. Why? I don't know. Now there are some tasks that the Win box is faster at but searching for files, switching windows/apps, launching/ quitting  apps is much more responsive on the mac. ( both have the same amount of ram)

Where you will notice speed differences is on apps that take advantage of hardware optimizations.
That is why photoshop was able to benchmark so high on slower mhz macs compared to top of the line pc's. If the software (such as poser) does not have these optimizations then the raw horsepower will determine the speed.
It's like saying that you have 8 processors at 2.6ghz
Its meaningless unless the applications can take advantage of them. Poser will run faster on a 4 core 3.0 ghz machine. has nothing to do with the max capabilities of the machine, rather the efficiency of the software.

Basically if you are tired of Windows it not that much more now to try Mac. If you are happy stick with what you like.