IF YOUR BUYING A MAC MINI PLEASE KEEP AN EYE OUT ON APPLE FORUMS BECAUSE THERE IS A HARDWARE CHANGE RIGHT AROUND THE CORNER!!!!
(sorry for shouting)
Seriously, I'm an audio production student (senior) and just purchased a mac last September for audio production specifically. Being a PC person my whole life I have to tell you that either route will work fine. I have Pro Tools 7.4 M-Powered on a PC of mine and compared to that of what I use on Macs in class there is not much difference (aside from my slower setup). BUT if you have the money to spend on a Mac, you will have the assurance that driver issues will be far less of a concern over time (apple can rollout drivers much quicker with less hardware concerns).
As someone else mentioned, a lot of what you might be concerned with lies in your audio interface. If you want some real in depth opinions on how well a Mini would perform, I would also post on apple.com's forum under mac mini or under logic pro. They're obviously going to be biased, but will be able to tell you whether a mini can handle a really heavy session.
Also, there are always ways around hardware short comings. For example, you can bypass any effect plugins during recording, and then while editing you can set your buffer higher and not worry about any issues.
If your planning on going with Cubase as you stated, again either PC or mac will work and meet your desired results. I use Logic Pro for all my personal stuff, but because I'm a SIU student I only pay 150$ for a 500$ program, otherwise I would go with Abelton or Pro Tools (and their semi-overpriced hardware ends up being worth it).
Ok I will end my rambling, I could talk this stuff all day.
