I hunt mid-September every year at around 10K feet in CO and I've always been happy with a 15 degree bag. Get a good pad, that's just as important. Thermarest Neoair is light and warm; I also take a lightweight closed-cell foam pad in 3/4 length like an Evazote or ridge rest. They're good for dragging out by the campfire or just lounging at lunch. And as a back-up if your inflatable fails.
