Boots tend to slide around in the bindings and a large portion of energy is lost through slippage and lifting the added weight. Skis on the other hand are quicker than snowshoes and, with the right waxes, mild temperatures are not a problem. Ski poles also offer added stability and balance. One of the drawbacks of skis is the need for an extra pair of boots. Also some areas accessible on snowshoes are impossible to reach on skis.
Falls can be more dangerous due to increased speed and clumsiness of a pack.
The sleeping bag should be your most expensive investment since it is where you'll spend almost half your time in the bush so don't skimp. Although more constricting than the standard rectangular shaped bag there is less air inside the bag that your body must heat up before you're comfortable and less bag means less weight and bulk on your back. Just remember that to get the equivalent warmth of a 5 lb mummy bag you would need a 7-10 lb rectangular bag. There are many excellent synthetic fills available on the market such as Lite Loft, Polar Guard HV, Micro Loft which offer admirable insulation properties, but for the best warmth to weight ratio goose down cannot be beat. A good quality down sleeping bag, well cared for and cleaned will last decades and lose absolutely no loft.