The Cinder +35 from Badlands is a really nice bag in that window and we've also got a few from Snugpak that would do the job. The Snugpak Elite series has a really cool feature with the expanda panel. It lets you literally expand the size of the bag when its warmer or when you just want more room. Also, their temp ratings are a bit optimistic, i.e. the Elite 3 is rated to +14f, but I would probably put it as a +20f bag.
If you're not going to be backpacking that really opens up the window, and I hate to admit it....but Campmor usually has some killer deals.