This question already has an answer here:
Is there any way to use an iPad mini or a Nexus 7 as a second monitor through a cable? I don't want something like vnc, I need it to work through the usb cable (or any other cable).
The reason why I am considering the iPad is because of the larger screen (compared to the Nexus 7). I think its self explanatory that I need this to work with Ubuntu.