I camp at similar times and find my western mountaineering bags rated at 35 and 20 cover all the bases. I have a highlite rated at 35 and a alpinelite rated at 20 degrees. I have taken the highlite down to 30 and the alpinelite down to 15 without problems. I would recommend a sleeping pad or combination of sleeping pads with a higher N rating if you plan on camping on frozen ground. These bags cover me for everything except my winter camping. Personally I find it hard to beat the western mountaineering bags. Oh and I also use my bags with a tarptent, specifically the original squall without problems.

Mike