You get much more of a room from a 40, plus in daylight that won't really be recognizable.
10mm is not that much a difference, especially from 40 to 50. 18 to 8mm would make much more of a difference.
50mm is also the focal length that most closely mimics the that of the human eye, giving photos a more natural perspective.
When you are shooting in low-light, you'll be wishing you had that extra stop.