Advantages of buying a complete ar, you usually already get everything you need to go shooting.
You can get a complete dpms for less than 600 before shipping and transfer fees or if you have an Academy near you that stocks them, you can get a S&W M&P Sport for 600 before taxes. Completely assembled, comes with mags, M&P comes with sights where as a dpms may not.
Advantages of buying separate, you may save some cash versus buying a complete higher end ar, like Colt for example, or some of the higher priced S&W's.
What I just did and am waiting on parts:
Complete lower, assembled - 190 shipped and then add 20 more when my FFL transfers it to me
Assembled upper minus BCG and CH - 290 shipped
Full auto, MPI, HPT, BCG and CH - 118 shipped
That brings everything to 618 total, but then I have to get mags as well but those will keep being purchased so I have not included these in my cost. It is 10 dollars more than ordering a complete dpms oracle including shipping and ffl transfer but I am getting what I feel are a few parts that are better quality so I can live with the extra 10 dollars versus that rifle and for the S&W Sport, my Academy's refuse to stock them and will not transfer one for me so I am out on that option, but if they did have one and I picked it up, I would have been at 650 out the door so my combo saved me 32 bucks.
This setup has what I want for my first AR and doesn't come with a bunch of crap I didn't want. Once assembled and tested at the range if it works like I want it to I will be happy.
If you are looking to save money then you can find some deals through sales going on with different companies and check out used items via gunbroker or forum classifieds. In my experience Midway has not always had the lowest prices so I usually shop around.
EDIT
Looking at the Midway site the only items I could find that fit those prices were both a dpms lower and a dpms upper. I'd shop around and look for a complete rifle.
NIB Oracle If you are looking, here is one example of a complete dpms rifle, 570 before shipping, that's already 75 dollars less than buying separate from Midway. You would still have to go through the FFL but that shouldn't be any problem. So figure shipping, lets say that it was an extra 25, and a transfer came out to another 50 you would be matching the pre shipping prices midway has, with everything at your door. Plus you get a case and 2 mags with it, so that's a minimum of 20 dollars in extras. In case you were wondering.