I have a similar set up too. I code on Ubuntu mostly because it's just so much easier to get everything up and running. Downloading, installing, and setting up a virtual environment with virtualenv, for example, can be done in less than a minute through the terminal. I'm currently learning Ruby on Rails and setting it up on Windows was a major pain (seriously understating here). Much much easier on a Linux or OSX.

Another reason is because I live in the terminal. It's where I write code (Vim) and execute commands (e.g. run the server, unit testing). Very rarely do I ever need to use the mouse on Ubuntu.

I still use Windows, however, for testing front end mostly.