Looking to hike the HMW in early September. Want to buy a bag that will cover a large range of temps for this trip and others. Seems like 20 degree is a popular rating. I'm very ignorant of the temp ratings in spite of the all my research. I'm looking to get the Nemo Riff I like a lot of the features, weight, etc. It comes in 15 and 30 degree versions. So my question is which rating would you all advise? Is a 15 degree to warm for a mid 40 night?

I am a warmer sleeper and like to wear a t-shirt, socks, and shorts. I have a complete layer of LW underwear if I need to warm it up.

Thanks in advance for the replies...