MacBook 13 or 15 retina. A great 13" for software development can be had for ~$1200.

Why Mac:

* It's Unix-like.

* Homebrew is fantastic for OSS package management.

* It's ubiquitous. (Yeah you could use Arch Linux instead of a Mac with Homebrew, but Homebrew will always get updates first.)

* Great battery life. (Linux often sucks at this.)

Why Retina:

* Text is extremely clear. It may seem like nothing before you've used it for a while, but you'll never go back.

* More real estate for side-by-side viewing of whatever you need.

* The Retina MBPs are actually quite light, enough so that I don't have MacBook Air envy ever.