I think I tend to use the 90mm most often for 6x17, unless I want to shoot handheld with the rangefinder on my Technika, which I can only do with a 150mm lens (it could be done with a 135mm lens, but my 135/3.5 Planar doesn't cover 6x17).

My DaYi 6x17 back doesn't have a red window, but simply an open window with a foam light seal around it and a shutter to keep the window closed when not winding the film, and this hasn't caused any problems.