I suppose this could apply to both Cubase 8 and 7.5 - Cubase on PC or on MAC, does one run better than the other?

No, but you can buy a really good PC for much less than a Mac.

Whatever you prefer as your OS. Usually Cubase (like any other cross-platform DAW) has slightly better performance on Windows (when run on the same or similar hardware), but difference is not big enough to justify switch from Mac to Win if you prefer MacOS as your OS.

However, like Romantique Tp said, you can get more power for given amount of $$$ in PC world. But if you prefer MacOS the additional cost may be justified. Your call.