Just to be clear: are you asking if we only play in one or the other aspect ratio (or both) or are you asking if our display is 16:9 or 4:3?
In any event, I have a 16:9 dispaly and I switch it up depening on how mangled the image gets when stretched (fast-scrolling horizontally-oriented games, for example, are messy in anything but 4:3 on a 16:9 display).