Well, what are your goals with the computer? Ubuntu is getting a lot more user friendly, but it's still lagging behind windows in media editing and gaming.
Ubuntu is free, don't ever pay for it. I dont know why, but I got this sense of you deciding for which to pay.
I've used Ubuntu for a few years now, and its taken me a while to get used to linux, you still have to do a lot of personal research as problems do arise often, and you do often have to do terminal work. I can, without a doubt, tell you that I have always relied on having a dual boot windows partition on the same computer. Unless you've used linux before, or don't mind reading a lot and learning how to deal with configuration files, go with Windows 7.