First off, I’d only ever view the water-resistant shell of a bag as only being ‘food’ or ‘spilt tea’ proof, and not storm proof. These shells are not taped and are really there to stop the down from sucking in water like a Pertex shell does (once its DWR wears off). These shells work well in damp tents, bothies and snow holes, but find yourself on an alpine route and you’ll quickly feel extremely exposed. Get caught in a storm and you’ll be fucked! A bivy bag provides more than just storm resistance (and that’s all it is, as you’ll only be able to sit out for one night before you start getting wet unless you find cover), it provides extra warmth (always said to be +5 degrees), and more importantly protection. On many bivvys on alpine faces you’re bivy spot can feature many sharp stones and spiky shards of granite, and you’re often jammed in some spot (not sleeping on a clean-cut ledge), meaning the sleeping bag gets shredded. Added to this on a sitting bivy your feet are often used to keep you in place, meaning the foot section of a bivy bag gets some hammer (you may be wearing boots inside your bag). A bivy bag tends to be slightly tougher than the shell of a sleeping bag so is easier to protect your bag, and any damage is easier to repair (seam grip). In bad weather you have no real way to keep crap out of your sleeping bag, as it has no cowl (most don’t), so bad weather (snow and rain), will have you soaked in only a few minutes. A bivy bag (with a cowl ideally), will allow you to create some space around your when sitting, letting you take off clothing and sort yourself out better (always get an XL bivvy bag for this reason), even use your stove inside. A good alpine bivy bag should be good for keeping off the wind (even light wind will rob an exposed sleeping bag of heat), rain, snow and spindrift. In really bad weather your chances of sitting out a really bad storm will be measured in only one night, and after that, you’ll either be going down or tackling some survival situation, the insulation you have going down by at least 50% a day (or more). In these situations often you’ll be depending on your clothing (this is why synthetic insulation is vital) and your bivy bag, rather than the bag, as this will soon be a damp rag (even damp it still has some insulation, but really only the same as two layers of material). The only way to extend your survival chances (or stave of survival mode) is to use either a bothy style bag or a light mountain tent (like a BD Firstlight). A bothy bag will allow you to cook and eat undercover, and keep the crap off your bags (the bivy bag only needs to keep the condensation off), while a tent can keep you relatively comfortable until your food runs out (neither extend your bag’s insulation indefinitely, only slow its degradation).
What’s the best bivy bag to buy? Well the XL Alpkit Hunka is the one I’ve used the most, but it’s not got a cowl, but is cheap (£40!) and light enough (500g). I would only go for a bag that’s breathable top and bottom (half and half bags are OK for camping, but not for sitting bivys). Rab makes some good bivvys but I don’t think their zip design is appropriate for alpine climbing. One of the best cold mountains bivvys would the PHD alpine bivy (not on their site but they will make you one), but it’s only snow proof (ask them to make it extra wide if you’re doing winter routes), but I’m sure they could make a taped waterproof bag if you needed one). The best bivvy bag I ever used was the Wild Things eVent bivy, but you can’t get them anymore, and so I made my own bivy bag out of Goretex. I guess the market is too small to produce something that specialised that will actually sell.
As for the sleeping bag element? Well, I think it’s important to address to overall use of the bag. Will, it simply be used on the route, with all your clothes on, where the nights will be short (going to bed late, getting up early), or will it be for every night of your trip? If you’re going to be sleeping in the valley, in huts, on glaciers, on summits and snow holes and in tents, then maybe a 200-gram fill is too light (I’d class this as a summer Pennine way bag). Also what time of year? Will it be August or late September? How tired and dehydrated will you be? What if you want to take one sleeping bag and open it up so you can share it, how will you do that on a ¾ length zip? On the hardest routes do you need a sleeping bag? How good are your moving together skills?
For me, the standard summer do it all bag would be around a fill of 500 grams of standard down. Add in high quality down and s tighter fit and you can push this down to 400 grams. For a woman, I’d go for a 600-gram bag, and make sure if they’re small that the bag actually fits them. Alpine and mountain bivvying is a real skill and takes a great deal of knowledge and experience to pull off, allowing a skilled person to bivy out in temperatures way lower than the bags recommended comfort range. Get it wrong and the opposite will be true!
For me I’d go for the extra 225 grams : )