You need to ask yourself how you plan to use the projector. If it is for games only or to watch standard television broadcasts, then 4:3 would be fine.
Personally, all of my DVD's are widescreen in 1.85 to 2.35 or more aspect ratio. Plus, I watch a lot of local HD broadcasts(16:9), so it would not make sense to go with 4:3.
If you want black bars to be at a minimum, then 16:9 would be best. Keep in mind though, that if you watch a standard 4:3 tv show or dvd on a 16:9 screen/projector, you'll get bars on the sides.
