If I were choosing between the MB Alpine Light Down Jacket vs parka, I'd choose the parka, though I chose the jacket last year before I understood how to keep warm in the backcountry.
However, I really like the detachable hood concept. One option is to buy a jacket and then supplement it with a separate down or synthetic balaclava type hood. The con is it's unlikely your hood and jacket will match exactly, but in the wilderness, that shouldn't be a big deal. The pro (for me)is that I can use the hood seperately with my quilt when it's too warm to wear the jacket as part of my sleep system.
Ideally, I'd get a Parka like the Feathered Friends Volant or Frontpoint, or the MB Permafrost Parka that comes with a detachable hood. But for now, I'm leaning towards ditching my Alpine Light Jacket for a NB Fugu and supplementing that with a merion balaclava + my BPL UL 60 balaclava.
If I had to choose between a MB Alpine Light down parka, or a MB Alpine Down jacket, the 1st thing I'd do is sit down and write a letter to MB asking them why, oh why, do you make hooded and non-hooded version of all your jackets except the Alpine Down? Then I'd probably choose the Alpine Down and supplement it with a hood, as I'm finding that my Alpine Light jacket and my MB UL Down Inner parka are not far enough apart in temperature coverage. The first covers me down to 35 or so, the 2nd from 35 down to 15-20, maybe, whereas a heavier jacket could take me from 35 down into the single digits, and layered with the UL parka, to zero.