I know a lot of people are suggesting MacBook pros and airs, which is great if you want both OS X and Linux, but.. apple = $$$. Part of what you pay for is OS X so if you want a Linux machine this is silly. If you don't specifically need Linux just Unix then I can't possibly recommend OS X highly enough. Also Linux distros and Mac hardware plays pretty well together but you WILL run into issues. My personal recurring problem is with graphics cards.

For a pure Linux machine the thinkpad is probably the way to go. They are cheap and powerful and most Linux distros are designed to work on them. Unfortunately it will feel like a cheap clunky pos next to any Apple computer and Lenovo may have put spyware directly in the bios where you can't do anything annoying to it........like get rid of it.

If you just wan't Unix go with a MacBook Pro. You do have to live with the price, and the fact that it is used primarily by hipsters and white girls for tasks one could accomplish on a raspberry pi (the new one at least). But the value of an OS written for the hardware is something that's hard to comprehend until you experience it.

Oh and get you one of them fancy new raspberry pi's as well. They're super dope, and they run Linux great.