It really depends what you plan on watching most. If you want to see "all" of the movie, go for 1.85. But then you will have some small black bars when you watch 16:9 material. Considering that feature films are shot 1.85:1 or 2.40:1, I would go for 1.85
I use a 1.78:1 and have been pleased with it. My understanding is that this ratio was selected to give the best compromise between various film ratios and also the 1.33:1 of standard TV as well as the 1.37:1 ratio of pre 1953 films. If you use a 1.78:1 screen you are also using the HD format. Also the 1.85:1 films fill the screen anyway if you have approximately 2% overscan.