With increasing age is coming increasing interest in bedding.
So what is the difference? If there is one, what are the arguements in favor of each?
There is no difference. Duvet is a European term for comforter.
A duvet is a slip cover that goes over a comforter (think very large pillow case). You get the benefit of keeping the actual comforter clean as well as being able to change decor by changing the pattern of the duvet material.
Sunglasses is right. Sorry, my mistake. In the stores the only time I see the term duvet is in relation to a cover (i.e., duvet cover) so the terms just stuck together for me.
Well, apparently the jargon is different here in Canada.
To wit:
A duvet is a big fat fluffy thing, typically stuffed with down - goose or eider, or a synthetic substitute. The duvet is plain - like a pillow, and must go in a duvet cover - like a pillow slip. These are very, very warm, and a good one is around $200 - $500, depending on the stuffing and density.
A comforter is like a comercially made quilt. Typically, they are synthetic filled, and fairly thin. The fabric on the outside is patterned - no cover is needed. They are a lighter weight item, and better suited for the summer months. You can get a comforter alone or in a Bed-in-a-Bag kit (You get the comforter, pillow shams, sheets, etc). Alone, they start at about $20, and can go up to about $125. With the kit they start at about $60, and can go up to about $350.
It would be usual to use a heavier duvet in the winter, and then switch to a lighter weight comforter in the summer. This is what I do, my mom does, and a number of my friends do. It could, however, just be a Canada thing.
Apologies to some posters, but the Duvet is the cover, not the comforter itself. It’s like a big zip-lock bag for a comforter.
My girlfriend will be so thrilled that I knew the answer to this!
Actually, I used to work for the distribution center for some home catalogs like “The Company Store” and “Domestications”. We sold a LOT of Duvets. They’re just the covers. Trust me. Big difference between a small, flat thing (folded, empty duvet) and a huge marshmallow-ey ball (folded comforter).
There must be some regional differences in terminology. I was just shopping for a comforter last night in Bed Bath & Beyond (here in NJ), and it went something like this:
Comforter - big fluffy thing, filled either with down or with synthetic fill.
** Duvet** - as bilgerat said, it’s a cover for your comforter. Most down comforters in particular are white (synthetics are more colorful) and you want to be able to protect your investment, as well as coordinate with your decor.
Quilt - about the weight of a blanket, these are mostly for decoration and not that useful for staying warm.
Other stores I’ve been to around here use the same terminology.
Switching between lighter and warmer comforters by season isn’t just a Canadian thing, though, because my family does it too. 
A comfortor/duvet is the actual down-filled blankie, the duvet COVER is the thing you put the duvet in.
I couldn’t care less what some idiot in Marketing decided that duvet ought to mean; it’s French, it means “down” (the kind you get from a duck), and since 1758, it’s meant comforter. The cover you put on your duvet is called a duvet cover. They call it that because “comforter cover” is too many syllables for your average marketing VP.
About $50 to $100.
Someone had to say it. 
I’m amazed nobody has linked to dictionary.com yet. And the answer is… a duvet is the big fluffy thing, usually encased in a [duvet] cover.
The difference? Uhh, pomposity?
I’m amazed nobody has linked to this:
According to Tyler Durden in Fight Club it’s a blanket.
I had this argument with my friend a few weeks ago. I said it was duvet/duvet cover and she said it was quilt/duvet. I’m so glad I’m right!
Another difference I’ve noticed is that a duvet/cover is often used in lieu of a top sheet. For instance on my bed, I have a mattress sheet, a top sheet and then my comforter. But when I was staying in England, everyone had just the mattress sheet, and then the duvet/cover.
Thanks folks! Whatever it’s called I know now that I want a warm puffy thing with a removable, washable, pillowcase-like cover.