There are a lot of options out there, making the decision a difficult one, but I suppose the first choice most make is whether to buy down or synthetic. I rarely go out for more than 4 days in the winter, so using down is not an issue for me.
However, I use synthetics for my belay jacket when ice climbing (DAS parka)- not so much due to a fear of saturating down, but rather due to the vast amount of duct tape patches on down jackets used for this purpose. If you will be using pointy things such as crampons, ice screws and tools, consider a synthetic just for the lessened issue of poking a hole in it.
Also, synthetics are not as lofty, meaning they might layer better with a bag (depends on your build, and the bags width).
With that said- all numbers be damned, but I have yet to use a synthetic coat or bag that compares in warmth to the same weight down item.
If going with down, look at the down load to decipher relative warmth. For a real winter coat (standing around in the teens) I like around 8-10 ounces of high quality down. A bit less if it has a good hood. So, for me the MB Alpine Light is not warm enough, with the MB Alpine being right on the border (and on sale at Prolite)
If you intend to be out of your bag alot, consider a hood as it makes a huge difference in warmth when inactive.
I lump down jackets into the following categories:
ultralight- less than 6 ounces of down, very light shell for above freezing)
sweaters- between 6-9 ounces, usually sewn through with no hood and a short cut. Good to maybe 35 degrees.
Parkas- 9-11 ounces of down, maybe baffled with a hood and longer cut. Ideal to the 20s.
Expedition- the monster puffballs.
If you intend to be active, then hop in your bag, a sweater would be fine. Otherwise, I'd get a parka. The FF Volant is an ideal example of the later (to me). However, for a sweater I'd get the MB Alpine at Prolite for under $140.