Whatever bag you choose, if in a base camp, buy at least 20 degrees colder than you think you need. The bag ratings are like a Candyland spinner on what score they use. Absolutely put a foam pad on cot to prevent cold coming up from floor into your back and kidneys, this is a must. I also found bringing a fleece throw can be insurance by placing in bag as an additional liner. Huge improvement in warmth. Buy bigger to allow more room for adding liners, wearing extra clothing plus more wiggle room. When you hit rack after hard day, you tend to move around a bit until you get "comfortable".
Watch camofire.com for sales on bags, all types and weather ratings.