Depends on the terrain and climate. Personally if I were in an area with a lot of bugs I'd go with a bivy, or one man tent. You can get one man tents at around the 2 to 2 1/2 pound mark. My Outdoor Research Advanced Bivy weighs in at 2 pounds. The nice thing about tents or bivy's when compared to a tarp is that you can have shelter above the treeline or in an area that's treeless.

A tarp on the other hand with likely weigh less, and have a lot of options in the way you can set it up. However, the tarp will depend on being in an area with trees to tie it off to. And if you're in a situation where there are a lot of bugs then you may want more protection.

Most of the time when I'm out in the wilderness by myself (hiking, huntin, etc....) I usually use my Hennessey Hammock. If the wife and dog are with me then it's a tent for us.

I've used a tarp to sleep out overnight in, it was alright half the time, and not so great the other half. In my opinion I'd weight the terrain, climate, and see what you're likely to face.