I'd also vote 10ー
You can always vent during the summer, but come fall if you're really beat up in the backcountry, under-fed, dehydrated, etc. you won't sleep as warm.
I sleep with a window open and fan blowing on me year round at home, but I've had several nights in the mountains where I sure was happy to have a 0ー quilt even though it was only 30ー or so. It's just tough to keep warm when you're pushing the limits in the hills for a few days. For an extra couple ounces it's totally worth it.
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