I went through this about a month ago. I settled on 2.35 (I have a Stewart cine-wide 133" with schneider a-lens).
Reason being - in my room, I wanted to maximize the width of the screen. Based on that, I looked at the various ratio's 2.35, 2.37 and 2.40. The 2.40 ratio screen was a little more than an inch shorter in height than the 2.35 (obviously) and I wanted to maximize the height of 16:9 content when not using the a-lens.
When I watch 2.40 aspect movies, I have a very, very small sliver of black bars top/bottom that I could easily zoom in a bit to eliminate (and spill out on the sides maybe an inch) but I don't even see them so haven't bothered. I like having slightly more height which I found to be a good compromise (and again, I could always zoom in slightly to eliminate if I ever wanted to). IMHO, it's a non-issue.