Here's my opinion.
If you are mechanically inclined, able to follow directions, and have access to the necessary tools (especially important when building an upper) then build one yourself. You'll get just a little more bang for the buck and you'll gain a deeper understanding of the AR-15. You'll also most likely build a full featured upper. Go into it knowing that you'll never get any resale value out of it. Would you buy an AR-15 that some guy put together?
If you are less mechanically inclined, and do not have access to the necessary tools (especially important when building an upper), then the M&P 15-Sport is the way to go. They're rifles built by a well known brand that has the resources to back their warranty. 1 year warranty / lifetime service policy. The trick is that you can't fiddle too much with the rifle or you'll booger the warranty coverage.
I purchased a 15-Sport for $550 back in the 1:8 5R chopped A2 rear sight days. I then got a bug up my pooper to try and build one for the same price. I did. Just beware, AR-15's can turn into fun money-pits. It can get out of hand quick if you let it.
My 15-Sport's gone out of control.
Earliest picture I have:
Current:
My "simple" home built isn't really that simple.
As it was:
Now:
Hard learned experience...
1. If you buy a 15-Sport, go put a few rounds downrange before changing things.
2. If you decide to build, check forums and websites for gently used furniture. You'll save some $$$. I'm the stupid guy who spent $$$ on a part, used it a few months, then got the bug to switch up. I unload parts from time to time, so do others.
3. While this is a S&W forum, don't discount other manufacturers.