Mike is much better qualified to answer screen related questions than me so you should get a more indepth answer from him. Although right now he's trying to finish up a TV review for us... so it might be a couple of days before you get an answer from him
In my opinion, any screen would result in a significantly better picture than projecting the image on the wall. The least expensive option would be to go with a fixed screen on the wall but of course this will have to stay permanently on the wall. A manual retractable option may be a more desirable option for you.
Here's a company that offers some very affordable screens: http://www.tigerdirect.ca. Just type in "screen" in the search box.