Minority opinion here: A lot of your decision depends on your theater configuration. We built a new room for our theater, so we had a lot of options - but with a width of 20' and a need for a door, speakers, and an equipment area, we were left with about 8' wide for a screen. A good deal of our movies are on DVD and are 4:3 format. We also have a Dish VIP 722. I wanted to do a constant width so I would have the largest image regardless of format. For me, that meant making the screen as wide as possible and using the 4:3 aspect ratio to determine the heigth. When watching a 4:3 movie, it is a 115" screen. When watching either 16:9 or 'scope, we still have an image that is 8' wide and the respective heigth. If we had gone with a 16:9 screen, the 4:3 material would have been substantially smaller. In fact, I would probably watch in on my 50" Panny plasma since it would be so much smaller than the 115" screen we have now.
Why not build the theater using a 4:3 screen and place the projector so you can use the zoom to fill the width regardless of format? That would give you the best of both! As an aside, I intended to use 2" wide duvetene tape for the black border - but after watching for about a year have decided that it would call more attention to the differing formats - so we still have no border. But we love the results and have watched over 500 hours of movies and specials over the last year, compared to maybe 6 to 10 DVD's in previous years.