A major problem/advantage you may have with the Sharp IR rangefinder is beam width. Unlike sonar, its fairly thin - meaning to detect an object your sensor must basically point directly at that object. Beware of chair legs! hehe . . .
So just how thin is the emitted IR beam? Well getting out my trusty IR detector thingy, its about this big:
The detector changes IR light into orange-ish light. Ok so that image is partly faked because my digital camera for some reason couldnt see the orange light, so I re-drew it in for you to see. The beam width is the same diameter as the lens on the left of the Sharp IR rangefinder. As the IR detector was distanced away, the beam fades and expands.