I think it probably depends mostly on you, and what bags you've used before. I've got a Western Mountaineering Mitylite (850 fill, 40 degree rating) that I've used in both quilt and bag mode for several years. When Thermarest came out with its Ventana quilt (650 fill, 40 degrees), I got one to try. It was really nice to sleep under, but I felt like it was heavier on me than the WM bag. It was a very minor difference, and I think I only noticed it because I had used the Mitylite first. If the Ventana had been my first down "bag" after using synthetic bags, I'd have been deliriously happy at how featherweight (pun intended) it felt to sleep under. (That was exactly how I felt when I got a TNF Blue Kazoo - probably a 600 fill - to replace a TNF Cat's Meow, the synthetic equivalent.)

If you're not used to an 850 fill bag, and not a fanatically picky gearhead (guilty, your honor) you'll probably notice no difference in use. However, the 650 fill will lead to a slightly heavier carry in your pack - the Ventana is half a pound heavier than the Mitylite; again, unless you're a total weight freak (I'm not "total" - yet), it won't make any practical difference on a long weekend.