Today, of course, technology has allows us to have it both ways-to tent, OR to tarp. But the way remains unclear at times. Myself, I'm a tent person. If I'm going to pack a shelter, I want it to be effective and reliable in all sorts of weather conditions. Tents have the advantage of a guaranteed dry floor, and walls that keep blowing rain out if the wind picks up. And a tent need not weigh a great deal-I often recommend the Sierra Designs Clip Flashlight ($189), a two-person tent (although better for one) that weighs under four pounds. GoLite's Den 2 ($199), which sleeps two and weighs just three pounds, eight ounces, is a real lightweight option.
On the other hand, tarps weight even less. Moss's Heptawing ($159) can cover up to three people yet weighs under two pounds. But then tarps only keep the rain off you when it's falling from above. You'll probably need a ground cover of some sort, in addition to the tarp. And tarps can be a little tricky to set up, though that also means they're open to creative interpretations. And you can fit tarps into places were you can't fit a tent.
So, for absolute minimum weight on a trip where rough, wet weather is apt to be the exception: Tarp. When reliable shelter is worth the extra two pounds: Tent. Thousands of years of debate are settled with these simple words.