Between the 9' and the 12', I'd choose the 12'. I have the 12' and like others have said, it is really versatile. It is a little heavy compared to the silnylon tarps, but much cheaper, especially if you find it on sale. I chose it for my first tarp because of that versatility. That said, I will probably add a smaller, lighter tarp this spring for use in nice weather.
Keep movin', keep believing and enjoy the journey!