I have heard the Mac is better geared toward graphics and music, but to be honest I will stick to the PC as software is more readily available, and it is less expensive.... although Mac users will disagree.

Faster processors and more memory equate to faster render times generally.

If you have a PC now, then you have to ask yourself do you want to learn a new system.

I suspect you would have to purchase Bryce for a Mac, as it is not just a port over from the PC. But then I also suspect that only DAZ can really answer that question.

It is possible to have a different login and so only specific software is loaded per login. That way the new PC (if it's that what you finally choose) can still be used for other purposes. Or I guess you could use your old computer for other stuff as well.
If you have a login that only loads the essential for Bryce for instance, well thats possible.

My experience with computers is that you will never be entirely happy, because when you find out how much faster the render times are with your shiny new machine, you will start to create more complex scenes and so the render times will become as they were or even longer LOL!

Best of luck choosing your new workstation.