WM Highlite vs. MB SSDH
WM is a bit lighter and maybe(???) a tad warmer.
MB has a much better hood and the SS makes it much more comfortable. For a sub-5lb exercise, i use the WM - this is about the only time i use it, though i might also consider it for a long day hike with a small pack as an emergency bag together with small volume custom made with mods MLD Epic fabric bivy.
For comfort and usability (full-zip allows better ventilation), i pick the MB bags. I've measured loft of both bags side-by-side and the loft on my comparable MB bag is the same as the WM Highlite-short length bag that i have. As i've stated the MB is a few ounces heavier (full-length zip vs. partial zip & the SS system adds ~2.0oz to a MB bag based upon a comparison of SS vs. Alpine (single/"inner" stretch only).
>>"combinations" and "permutations"
Basically, in one sense, permutations are arrangements of elements/items of your set, whereas combinations are selections elements/items or your set.
So, it's probably "combinations" that you mean, unless you don't care whether a particular bag (e.g., the down bag) is used as the inner bag or the outer bag (arrangment, hence "permutation"). If the down bag is only considered for use as the inner bag, then "combinations" (a selected bag can only be used in one inner or outer position - position isn't considered a counting criteria in combinations).
So, if you have four bags and are selecting any two of them, you have six possible combinations. If you are selecting two of the four bags and also considering how they are arranged (inner & outer, -OR- outer and inner positioning being considered), then you have 12 arrangments or permutations of your 4 bags. To quote Mike Barney, it's really a "simple" problem (and this one really is. i'll leave the non-simple problems to Mike).
>>"half a pj"
very funny. exactly what i was trying to avoid. Wait, "half-a-pj", hmmm...you know, that could work, it would be sorta' like a penny and a "DrJ" or a "Ryan" unit of measure could be like a $100 bill. Maybe, we could accumulate "Ryans" and use them to obtain "bennies" in the BPL.com store. A SpinPoncho costs 100R[yans]. Also, maybe, a "pj" might be a wee negative unit of measure that detracts from me pot-o-"Ryans" that i be accumulatin'? Just a [bad] thought and even worse attempt a humor!!