I use my medium format gear in the winter in the mountains of the American Pacific Northwest and have owned Phase One, Sinar and Hasselblad digital backs. Currently, I own a Leaf back.
Digital backs are extremely power hungry in the first place, compared to your small format camera. You will need to bring more extra batteries then you are used to when you are away from power. In the cold, the batteries do run down more quickly, but not excessively. I hope I am not cursing myself by saying this, but I have never lost a shot in the wilderness due to a lack of battery power. It is something that is relatively straightforward to plan for. Just be aware that bringing many more batteries than you are accustomed to will become the norm.
If I have a busy day of shooting, I can go through three batteries a day. If I actively manage my battery power (which I usually do), I can cut my battery usage rate by 2-3x.
As for dust, the digital back sensor collects more dust than the small format cameras. I believe the interior of medium format cameras is signficantly dustier, and the lack of self-cleaning sensor technology contributes to this difference as well.
Still, with the digital back sensor so readily accessible, cleaning is not problematic. If my gear is clean when I depart, I am usually fine for the duration of the trip, provided my back is never removed, and I am diligent about not exposing the interior of the camera to the elements. At the end of such multi-day trips, there are usually a few minor spots that need to be removed, but nothing that requires significant work. A handful of times, things have gotten bad in the field--cleaning in the field in a sheltered environment (cave, tent, etc.) was not a problem.
You'll find that medium format is signficantly more light hungry compared to small format than the specs first suggest (you'll be using slower shutter speeds to get enough light, since lenses are slower, maximum usable ISO's are lower and digital back's native ISO's are generally lower than in small format). But if you've test-driven a setup and find you can accomodate the differences in use, handling and performance, you should find yourself very pleased with the resulting boost in image quality.
I'm not sure if you are on the mountains you are photographing, but alpine photography is probably one of the few places outside the studio where you often have light to burn--if you're above the clouds, on a glacier, as you probably know, there is so much light that your eyes need protection from it. Medium format cameras are very happy there...
On the whole, it's difficult to find 'night and day' differences between "digital" lenses and their predecessors. Of course, there are specific cases where this is the case, but by and large the differences are not radical. One strategy might be to compare the price of a digital lens with its non-digital predecessor. If the price difference is substantial, ask for sample images (full-res, or even raw) from both lenses, from the back you're planning to use. Often dealers can provide this, as can the community here at LuLa.
Then, and this is important, trust your eyes. There will be a lot of marketing material and opinions from the sidelines as to which is better and advice on what you should buy, but in the end, trusting your eyes and your knowledge of your intended use will tell you whether or not the digital lenses are worth the premium.
Let us know how it goes,
