It's probably best to begin with the 2x while you're getting familiar with the camera. It's also much more difficult to center a planet in a narrow field. With the 5x Jupiter takes up at least 1/3 of my screen as far as I remember, that doesn't leave much wriggle room. Depending on your tracking accuracy you may have to adjust quite often.
Once you are ready for better images then the 5x will be invaluable though. I got mine from OPT:
http://www.optcorp.com/product.aspx?pid=194