Pros:

• The zombies can't reach you.
• You can get a safe night's full rest.
• The leaves from the tree(s) you are in can block you from view.

Cons:

• There's only one way out: Down. How you get down is optional, but no matter what, you can be effectively boxed in. A loitering zombie could a) Delay your departure or b) Force you to kill it, drawing attention to you.
• If a raider spots you, you're toast.

Best way of accomplishing this: IMO the best way to sleep up a tree is by stringing a hammock high up between two tall trees, or between two branches. You do want a lightweight hammock that you can carry for as long as you want, though. A good example of one such hammock can be found here. The adaptations that you can make to it to make it bug proof/rain proof are cheap and easy. You can also find information about sleeping in a hammock here.

Pros:

• You're very well hidden.
• There are multiple was of escape, if need be.
• Nobody (Not even a zombie) has any reason to walk around a thicket.

Cons:

• Your escape will be hindered, if you have a need for one.
• If somebody knows you're in there, the only thing you can really do is shoot him while he's still trying to figure out your exact position.
• It will not be a very comfortable night's rest.
• Some other animal might already be living there. Maybe even a nest of fire ants/hornets. Thinks about that one.
• If it starts to rain . . .

Best way of accomplishing this: I'd say just crawl under the bushes until you find a space large enough to lie down somewhat comfortably in, and then just fall asleep. Hopefully nothing will crawl into your clothes while you reside there for the night.

Pros:

• It offers reliable and high quality shelter.
• It's bug-proof, waterproof, and wind proof.
• We make them for a reason.

Cons:

• You can't really hide a tent.
• It takes a while to set up/dismantle.
• When inside it, you can't see what's going on with the rest of the world. A zombie could sneak up on you no problem.
• If a zombie does sneak up on you, it takes a few seconds to get out of the tent and get situated. And then what? You've left your tent behind.

Best way of accomplishing this: Try to find the hardest-to-acces, most-camoflauged-by-shrubbery place to set up your tent. Make sure to scout the surrounding area visually for any immediate danger.




Sleeping in a Bivy Sack:

Pros:

• Easily concealabe.
• Small, lightweight, and warm.
• Probably the smallest form of shelter you can get.

Cons:

• As with the tent, visibility of the outside world is null.
• Also as with the tent, it'll take a while to get out of the thing if you need to move fast.

Bet way of accomplishing this: Buy a small, lightweight bivy sack (Here's a good one - Expensive, though). When setting it up, look for a clump of bushes that you can set it up by to provide some camoflauge.