Want to try Ubuntu and still having Android? You are in luck now. Canonical has an application to let you dual boot both Ubuntu and Android. Switch between these 2 OS via Ubuntu Dual boot app. So far it is working and tested on Nexus 4. Other Nexus devices like Nexus 7 & 10 should work but not tested. Therefore, try it out if you want.
Interested? Head to Ubuntu website (link) to get the installation guide.