I would think that the hoops provide more headroom near the perimeter? More useful square footage/yard material? Just guessing really.

A glass room seems an ok concept - my thought is that the utility (warmness) would be well compromised without a roof, although maybe good for the cool crops in a borderline temp situation.....but I would see a glass room as 75% of the work and expense of a greenhouse for only 15-20% of the utility....

I don't see a zone in your post - so that might well have a large impact. Maybe a glass room in zone 7 or 8 would do the trick....not in zone 5.

You loose heat by radiation and convection. Wind is the convection part, so if you saw the art room on a sunny day (minimal radiative loss) and it blocked the wind, it felt good. But the radiant temperature of the sky is very cold. Just walls won't help much at night.

The reason people cover beds with hoops is that they're simple and easy. A 10' stick of conduit can cover a 3' wide bed. It's relatively cheap and easy to bend into an arch with enough left to push each leg into the ground a foot. Cover it with both fabric and plastic and you can overwinter onions in zone 5. The plastic stops air circulation losses, and the fabric moderates solar gain for even temperature.

The drawback of hoops is that as size goes up, snow load becomes an issue. A greenhouse operation near me lost several 20' wide tunnels to snow last year when we got about 15" of snow in one dump. That's why gothic arches are better for snow.

As size goes down, hoops are hard to get into. You have to take the skin off to get in there. Single bed hoops are best for things you're going to mostly leave alone over the winter.

I like my wood truss gothic arch greenhouse. It's easy to get into, cheap, repairable, and was easy enough to build. The frame cost me about $100 in strapping and screws, and the rest was scavenged material. I was even able to re-use some greenhouse plastic to cover it.

http://img.photobucket.com/albums/v219/huisjen/IMG_4029.jpg

For extra protection inside, you can lay row cover over the crops. You can lift the row cover easily for work or harvest, and being inside, it won't blow away.