I like Ubuntu and used to use it as a secondary OS alongside Windows; however, AMD cards made the experience a nightmare. I'm a fan of their products, but their poor driver support was simply unacceptable especially in light of how well Linux works with Intel and Nvidia. I have a Radeon HD 7750 now which I couldn't get to work with Ubuntu 12.04 for the life of me. It was around then that I decided to abandon AMD and opt for Nvidia for my next card
However, I've been hearing all this talk about mantle and also how they're committed to getting their act together with linux driver support in preparation for Steam OS. I had my eye on getting an Nvidia 650 TI boost, but should I wait to see what happens with AMD support in the next few months?
If you really want something that working right now just get nvidia. If you're willing to wait then wait. But personally i don't know if mantle will ever come to linux or steam os. I thought that the point of valve with steamos was to push open standards like opengl