So, the question becomes: What bag? As a rule of thumb, a bag rated to -20 degrees would be about right. Good down-filled candidates include Marmot's Col ($599), Feathered Friends' Ptarmigan ($550), or Western Mountaineering's Big Horn Super MF ($530). All are tremendous bags, well made, with high-loft down. Marmot and Feathered Friends use a water-resistant shell material; Western Mountaineering a breathable polyester that I think is a bit better for super-cold weather.
In the synthetic camp, I'd recommend Mountain Hardwear's Fifth Dimension ($325), rated to -25. But, it weighs nearly seven pounds, versus about four pounds or less for the down models. So that's the penalty.
My advice also is to take a vapor barrier liner as a supplement if needed. And two pads - an inflatable pad such as a Therm-a-Rest, and a closed-cell foam pad. And take both with you up to 17,000 feet. Even though you'll be tempted to save a pound and leave one behind, you'll be sorry if you do.
The down suit question again depends on time of year. For May, full down will be extremely useful. When I climbed the West Buttress in June, I was fine with heavy Polartec pants under Gore-Tex bibs, and a Feathered Friends down parka. The suit versus jacket/pants question is sort of a matter of taste. A suit is warmer and weighs less than a jacket/pants combo, but is less versatile. Generally, I think separate garments give you more wear flexibility, and almost the same performance as a suit.
Good luck! Let us know how you do.
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