I have a piston driven AR, now i'm wanting a DI as a second rifle. I know nothing about AR's and need some help.
What i'm looking for is the pros and cons between the higher end and lower end uppers.
Should i buy complete or build? Budget is about $1200.
 

Buy 

Can't say I'm any kind of "expert" but I'm in the middle of my first build right now.
With a budget of $1200 you could buy a pretty nice complete upper. The only advantage to building would be that you could go with much higher end components at reduced cost.
The caveat is, you'll need someone experienced to help you with assembly. You definitely want to get it right the first time.
Check out rainierarms.com, they have some pretty nice stuff, and fast, flat rate delivery.

Even if i buy i still take it apart and put it back together to make sure everything is in spec.  Even big $$ uppers i have found things that are not correct or to my spec.

Number 1 thing i find is loose barrel nuts or not tight enough.   If DI gas tube alignment

I'd  buy upper. Uppers require specific tools and depending on the type of rail you would need proprietary barrel nut wrench (eg. Knights is $300 for URX 3.1)
BCM has great uppers for <$1000. Other companies have even cheaper.

You'll need to headspace the upper with a set of headspace gauges and an AR spanner.

If you are new to assembling an AR just buy a complete one.

While checking the headspace is a prudent step when building an upper, I've read many articles and watch several videos saying if you buy a good-quality popular-brand barrel and upper receiver that meet specs, the headspace will be within specs as well.

I'm not advising anyone to not use a gauge, but the odds of the headspace being wrong are very low if you choose quality parts.

Always better to be safe than sorry, just saying the odds are in your favor if you choose the right parts.

At least one no go gauge.
Its so cheap at $10 well worth for a peace of mind.
I do so many builds that it does come in handy.
But for one build and a budget of $1200 , can't see why not .

Go buy a colt LE 6721 for $900. It's a proven rifle that will shoot just as good as a $2000 rifle.

Here's a good maker of uppers
http://www.laruetactical.com/uppers?sort=popular

and another one I had experience with recently:
http://www.keystoneaccuracy.com/home.html

I doubt if you will need to take apart their uppers to check for any loose barrel nuts or to check if parts are "in spec".
Larue's capability and experience are many times more than anyone here.

Keystone pinned a rear sight and milled an upper for me. Got it back in a week. Good work.

While checking the headspace is a prudent step when building an upper, I've read many articles and watch several videos saying if you buy a good-quality popular-brand barrel and upper receiver that meet specs, the headspace will be within specs as well.

I'm not advising anyone to not use a gauge, but the odds of the headspace being wrong are very low if you choose quality parts.

Always better to be safe than sorry, just saying the odds are in your favor if you choose the right parts.

Totally agree on "always better to be safe than sorry" and I personally would insist on gauging it...some things I just don't take for granted regardless of the odds and even the very best of manufactures spit out a dud every once in awhile.

Price is negligible, just pick an upper you like from a respected company.

The biggest benefit for me is not having to drop all the money at once when you build. Gives me the impression I am saving money.

Thanks for all the info. I was just curious as to why there was a huge swing between prices. I'm seeing uppers from 399 all the way up to 1600ish. At first i thought
it was just name brand hype, but it seems you really get what u pay for.I also thought maybe i could get a better deal building my own vs buying.
Seems to me that the higher end of the scale would be the way to go since i don't have the specialty tools needed to build it my self. 

The cost to getting the tools more or less equalizes the cost of buying a pre built upper. Also consider shipping charges if you don't get all the stuff at the same time or from the same place. It makes sense (to get the tools) if you anticipate building a number of uppers. Take a look at BCM, they have a number of configurations. With you're budget you've got a lot of possibilities though. One to avoid is PSA's PTAC uppers, have seen some pretty sub par products come out from under that PTAC name.

I have a piston driven AR, now i'm wanting a DI as a second rifle. I know nothing about AR's and need some help.
What i'm looking for is the pros and cons between the higher end and lower end uppers.
Should i buy complete or build? Budget is about $1200.

since you already admit that "I know nothing about AR's and need some help.", I'd highly suggest buying an upper.  Screwing up the headspacing is a huge problem you don't want to have to deal with!  Yes you may want to still check head spacing on a prebuilt upper but chances are your much less likely to get a screw up prebuilt upper which has been test fired for quality control rather then building your own and not having all the necessary tools as well as not knowing what you are doing.  Personally I have yet to find an upper parts kit which is less expensive then a prebuilt upper.

As far as uppers from 399 all the way up to 1600ish, the question is how accurate do you need the rifle to be, what accessories do you need (do you want/need a free floating carbonfiber rail or quad rails to attach all your goodies), and how light weight do you need it to be?  Most uppers will do 3/4-1.5moa out of the box unless something is way out of spec.  Most of the time, better ammo and an expensive/better trigger will make a bigger difference in your groups if you are going for accuracy.  If you need something with 1/2MAO or below, its gonna cost you a bit more (in either weight or price).  If you just need a rifle to run and gun as well as not be afraid to drop or to have fun rapid firing at targets with in 50 yards at fun shoots, you don't really need a 2000 upper.  Yes it would be nice but a spare 400 dollar upper would work great for you if you don't want to wear out/scratch your other/nicer piston upper. 

You guys know the head space on an AR is set when the barrel extension is installed by the barrel manufacturer, right?
The tools required to assemble an upper will cost about $100.
If you are reasonably mechanically inclined, you can do it with the procedures you find on the internet. There is the military m-16 manual and loads of youtubes on it.
If you are worried about headspace, you can get matched bolts and barrels from shilen and others. Krieger will headspace a barrel on your bolt if you mail it to them. But the fact is, the parts are standardized, I have never had more than 2 thousandths variance in headspace in my AR's. Headspace gauges are about $20 a pop and you need 2 or a go and some tape. Or you can assemble your upper and have a gunsmith check it for you.
You just need to decide whats best for you; the pride of having built your upper or the pride of having a expensive brand name upper.

headspacing can vary from bolt to bolt.  As with barrel to barrel.  Most barrels are spot on. Plated bolts will vary a bit.

You guys know the head space on an AR is set when the barrel extension is installed by the barrel manufacturer, right?
The tools required to assemble an upper will cost about $100.
If you are reasonably mechanically inclined, you can do it with the procedures you find on the internet. There is the military m-16 manual and loads of youtubes on it.
If you are worried about headspace, you can get matched bolts and barrels from shilen and others. Krieger will headspace a barrel on your bolt if you mail it to them. But the fact is, the parts are standardized, I have never had more than 2 thousandths variance in headspace in my AR's. Headspace gauges are about $20 a pop and you need 2 or a go and some tape. Or you can assemble your upper and have a gunsmith check it for you.
You just need to decide whats best for you; the pride of having built your upper or the pride of having a expensive brand name upper.

this 

http://www.amazon.com/exec/obidos/ASIN/B00AUEHR98/?tag=2ahawaii-20

pretty good starter builders kit

http://www.brownells.com/.aspx/lid=11004/guntechdetail/how_to_build_an_ar-15_video

very good how to videos on building. helped me while building my last AR.

Just take it to Dex...I wish I was still on the island, as I have some things I would want him to do

Thanks for all the advise. I ended up buying a Noveske upper from OGC. Next one will be built,
Hopefully!!!