Well, my opinion, based on my experience but no real data, is that more breathability helps somewhat with condensation on the interior of the shell fabric, but is much less important than having the overbag thick enough and the inner bag thin enough. there are two factors in play when it comes to dew point; temperature and humidity. The insulation amounts are critical to getting the dew point out into the synthetic layer. Once you achieve that, then, and only then, the breathability of the outer shell can come into play as a way to maybe reduce humidity within the whole assembly. But I expect that if you are going to use the kind of fabrics I would be thinking of – as in, the lightest I could find, you will have a pretty narrow range of breathability to choose from, and quite possibly no solid information on the relative breathability of the fabrics you are looking at. So my approach would be to get the lightest thing I could find; and if by chance I did have numbers on breathability, and I had two fabrics close to being the lightest, then I would take the one with more breathability.
My own outer bag is 2.5 oz climashield, .67 argon inside and out. I’ve had that work pretty well both for warmth and for moisture management in the low 20’s and high teens over a bag that keeps me warm to the mid 30’s without it. if you use AD with an outer shell then you would have probably less insulation but more breathability since you only have one shell layer.
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