Those conditions pretty much take canister stoves out of the equationthey lose performance as the weather cools, and even the "good" ones, with a lot of propane, aren't worth much once the temp hits single digits. Now, white gas has always been a cold-weather performer, at least when I've used it in below-zero temperatures. Go with the tried-and-true here, something such as the MSR XGK ($110; www.msrcorp.com), a classic expedition-type stove that really works well when it's cold. The Optimus Nova ($149) is another excellent cold-weather stove, and although it's expensive as hell, it's incredibly ruggedly made, so will last for many, many...many years.
There's one other option: A stove that burns straight propane. Propane burns well when it's very cold, and has tons of BTUs. The downside is you have to lug those big canisters (no, not the ones that attach to a barbecuethe green bottles that are about the size of a football). But one lasts several days.
Coleman used to make a "backpacking"-style burner with an attachment that screwed into a propane bottle. Alas, no more. Century makes one that screws directly into the bottle, with a plastic base to keep it stable. Not that stable, unfortunately. Still, it's worth a look, and the stove itself is only $25 or so (www.centurycamping.com). Otherwise, go with the XGK or Nova. Either one will keep you in hot chocolate no matter how cold it is.