Compare the 644 Tents on Ten Pound Backpack based on their weight, price and a few more features as well.
Most campers, backpackers and campers go assume that they need a tent, without ever realizing that there is another very viable option for keeping the weather away from you while you sleep, which is the bivy sack. The bivy sack vs tent debate is a good one to have, because most people come out of it realizing that bivy sacks are actually a great option, with a lot of positives in their column. For one, they weigh a lot less than a tent, and take up a lot less room in a backpack.
First, it has to be said that when it comes to weight, bivy sacks easily win out. Even a heavier bivy sack will be in the same weight range as only the lightest tents. So in terms of weight, point goes to bivy sacks. The Marmot Alpinist, for example, weighs just 14 ounces, or 396 grams, and it is far from the lightest bivy sack out there.
Also in the bivy sack column is the footprint. Some tents need quite a large clear, flat area to set up, but a bivy sack requires very little space, and does not need good ground for tent pegs and the like. Even bivy sacks that come with a pole built in, like the Outdoor Research Alpine Bivy, have a very minimal footprint.
The major failing of bivy sacks is waterproofing. It would be easy enough to make a truly waterproof bivy, but then the interior of the bivy would get more than a little damp, and you would end up getting wet from the inside, rather than the outside. Materials like Gore-Tex are good, but not good enough in something like a rainstorm. Adding a tarp to your setup helps to solve this problem, but also adds weight and bulk, the lack of which is one of the big advantages of bivy sacks.
Where bivy sacks lack most, in protecting you from the weather, tents excel. Even a simple, single wall tent does a much better job of keeping you dry and well ventilated. And there are also some very lightweight 2 wall tents out there that do an even better job than single wall designs of protecting you from the world outside.
One of the biggest advantages of tents that bivy sacks cant hope to touch, however, is the ability to fit more than one person inside. Two or three person tents help to save a lot of weight, and make camping and backpacking a lot simpler. For a group of three only one three man tent is required, but three bivy sacks are required.
Where tents fall short is in the weight category. Even the two very lightweight tents above are no match, in terms of weight, for even a moderately lightweight bivy sack. Tents also have a larger footprint, and while this may not be a problem, depending on where you're setting up camp, it will matter to some.
Bivy sacks are great when you don't have to worry about the weather. If you need a just-in-case shelter, they're ideal. But when the weather gets bad you're likely to be better served by the extra protection and space of a tent.
You can find a lot more tents and bivy sacks to compare and contrast over at the compare bivy sacks page and the compare tents page.