^It all depends on what you want to do with it. The first one I bought was the higher-end one available at the time, and I got it because it had the USB jack. But I rarely used it, and frankly, it didn't work very well in recognizing the files I wanted to use it to view. What I have now is the cheaper purple one (2 of them, actually). I found the newest incarnation really tempting, as the remote includes a headphone jack. I would love it to be able to run music apps and walk around the house with headphones in my ears. It's marketed as being great for watching late at night without disturbing someone else, which sounds great except that I live alone and don't have to worry about that.
Glancing at the product list, really the choice is between the least and most expensive models. If you don't need what the most expensive one has to offer, buy the least expensive purple one. Plus you could get two of them for the cost of the one higher-end model, which you WILL want if you have more than one TV, because you will love it and end up watching all your TV in one room.
At least, that's what happened to me.