The difference between 32* weather and 39* weather is dramatic. If you were only expecting the latter, i'd say use the 50, but since the former is a possibility, use the 40. Of course, there are a few variables. If you sleep outside a lot the month before the trip, you can adjust your tolerance and get away with less. Your sleeping pad and shelter are also relevant.
If possible, test! I'm becoming a masochist when it comes to cold temperatures, and I always try to see what my lowest tolerance is for warmth. Over the course o the winter, my body adjusted dramatically.