Which ProdigyX temp?

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.

If you added 6 oz with the 40, you'd be able to shed many more ounces by not bringing full fleece sleepwear, especially fleece pants (useless during the day). Use a super light down jacket and some synthetic tights like Under Armour.

The pants weigh 11.2 ounces (medium size – my weighing), and it would be a lot lighter to simply get a the 40 degree bag and ditch the pants.

I was trying to decide the same thing a couple of weeks ago and I decided to go with the 40* quilt. I noticed that the 40* quilt used 4.0 oz Apex Climasheld, and the same quilt last year was considered a 35* quilt. I figured the 40* would be the most versitle. Add a bivy and some extra clothes and should be okay down to freezing and if its really warm, it can be opened up flat and used as a blanket. I know you can save 6 oz with the 50* quilt, but when you are laying there shivering on a really cold night, 6 oz might not be a really big geal.

I agree with the others, go for the 40F as temps that time of year are unpredictable and better to have a little more versatility. You could save the wight by going with the RevX, it's a fantastic quilt.