what is the best brand of computer to use for unity

They're about the same so pick your favorite platform. If you want to develop for iOS then choose a Mac because you'd need to compile your project on the Mac OS-platform.

Its more of a personal choice and what kind of projects you'll be working on

well i will say i depends on what os your use to(though their is not much of a deference),all so some programs are windows only and some are os x only,so in hindsight it depends on what programs you need.i will be using windows xp since it has what i need.(however most say mac is better)

ya pretty weird question, there no real difference, and its not the brand ur talking about the Operating system. but yea in short if u want to build ur project for iPhones u need a Mac pc to build the project. thats about it.

Don't forget Mac's don't support 3DS MAX natively - too some people this ain't a problem- but me personally 3DS is the best 3D modeling software out there - by a whale. (You'll be stuck with Maya :C)