I am buying my first ar and i am unsure as to which barrel length to get,16" or 20". pro's and con's??

You cant go wrong with either.  
I got a 16 inch. A little bit lighter.
Whatever feels right in your hands, if what you should pick.
Or if you start saying in the middle of the gun store, "this is my rifle, there are many like it, but this one is mine!"[;)]
c-rock

What do you plan on doing with your new AR?

What ranges do you plan to shoot?

What type of ammo do you plan on shooting?

What kind of accuracy do you want to get out of your rifle?

Is the price a big factor?

Lots of questions, but, there are lots of factors involved. You can almost go bottom of the barrel, and buy a throw-away rifle, or, you can go with a top-of-the-line AR. Most folks find themselves somewhere in the middle.

Shop around, and get the best deal on exactly the rifle that you want.

16" will be more for a close range(0 to 300 yds), rounds on target rifle. Easy to move in close quarters, lighter.

20" will gain you some accuracy(, and allow for longer shotspoint targets well past 500 yds)

Another beauty of the AR is that you can buy another upper to add to your system. So, you can buy a 20" upper now, and add a 16" later, depending on what you decide to do.

Lew

the only advantage to a longer barrel is more velocity for longer range shooting.

[b]Bushmaster Dissipator[/b]:

Shorter bbl. with longer sight radius.
Perfect.

my wife keeps telling me that sized doesn't matter....