What BD tool is best for winter climbing?
I climb leashless a lot--probably about 80 percent of the time. Leashless climbing is sort of like telemark skiing vs alpine skiing: You might not have quite as much security on tele gear, but the freedom and grins are worth it. Leashless climbing offers a lot of advantages, especialy for hard mixed routes and ice cragging. I wrote extensively about this in Ice and Mixed Climbing, but the very short version is that I go pure leashless with Fusions when on hard mixed routes, "trad" leashless when cragging with Viper Fangs, and leashed when I want maximum effeciency and security on big lines. Here are a few thoughts on tool selection:
The Fusion was designed primarily as a "hooking" tool. It swings well enough once you adapt to it (trying swining it with your thumb on top of the horizontal handle part, that works really well) and is the best mixed tool I've ever climbed on, but it wasn't designed for swinging all day long (although some people seem to do fine with it). The more weight in the form of grips and so on on that's in the wrong place (the handle) the worse a tool will swing. It's basic physics, and if we want good grips and places to match then swinging performance will suffer to some extent. However, unlike all other current leashless tools, the Fusion shifts only very minimally when changing grip positions from the lower to the upper and back. Hang from a doorframe on the tool and check this out. This "shift-free" feature means you don't fall off when matching on a tool balanced on a micro-edge. There are a lot of matching moves only possible with a Fusion.
The Viper Fang is the next step up in security. I find it easier to hang onto than the Fusion, especially with gloves on, and it still swings very, very well. A Viper is a good choice if you primarily climb ice and may want to mess about on the mixed some. You can still use Androids for big routes, yet switch it out for running laps in the ice park, climbing ice at a grade you're bored silly with or anytime you don't need to swing thousands of times in a day. I climb on Viper Fangs a lot, it's a good compromise.
The Cobra or Viper with an Android is heavy artillery for moving well up fresh ice. Nothing swings as well as a Cobra or Viper with a leash, and I feel way more comfortable 30 feet above my last screw with a leash for extra security. I might be old school, but I don't fall on ice, I've seen way too much carnage over the years.