In other words, in its purest form, a bivy bag was a light, one-pound Gore-Tex bag you stuck into the bottom of the pack with the intention of never using it. In recent years, though, bivy bags have morphed into surprisingly complex, heavy, and expensive mini-tents, and a lot of people ask about using them instead of a tent. And it's true, they still save some weight - Outdoor Research's Deluxe Bivy Sack ($249) weighs one pound, nine ounces, and has a single pole to hold the thing off your head. Once in a great while I'll use a bivy bag when I'm really paring weight and don't plan to be out for more than a night or two.
But comfortable? Fuggetaboudit. Try this: Get a plastic garbage sack the dark, opaque varietyand stick it over your head (OK, cut an air hole first). Got into the shower and turn on the water. Now sit on the floor of the shower. For hours. You'll get the picture - no reading, no card playing, nothing that makes the long hours stuck in a tent remotely bearable.
So, for that reason, I frown on bivy bags for everyday camping. Besides, while it's true a bivy bag weighs two pounds or less, many good tents weight only three pounds per person when the weight (parsed into poles, rain fly, and tent) is split among two or more. And for that extra pound, you gain tons of comfort.
So I say get a tent. You'll find it more useful, more comfortable - all-around, a better investment. Plus, decent two-person tents start at around $150, far less than bivy bags. And for another $100 you can get a classic two-person, three-season tent like Marmot's Nutshell ($249), which weighs less than five pounds, putting it very close to bivy bag territory in terms of weigh for two people.