I have a PC (Intel i7) and a Macbook (Intel i7, 2011) - and they both run Cubase 6 and 7 just fine. I even exchange projects between the two platforms on a regular basis…no problem…at all!
If you are using lots of samples I would focus on a fast harddrive, more ram etc. - maybe even an SSD (depending on your budget).
That being said I’m sure that a new Mac Pro will be a great machine that will be fine for years. Only question is: when will Apple release a new version?