Along with reasonable prices, this video pretty much sums up why the pmag is the go to brand and is trusted by almost everybody.
http://www.youtube.com/watch?v=OiN_29WDFhU
Buy a hexmag and try it out, they're only a few dollars. If you like it and it doesn't give you any issues get some more, I stick with pmags, but that's just me.
http://www.youtube.com/watch?v=8ynHHSnKUy0
At a quick glance, the M&P 15 (811000) seems to have a different barrel profile than the Sport 2, a chrome lined bore and chamber and a carry handle.
I guess that's where the $ difference is. The trigger guard is different too.
The Vortex Sparc 2 is a very nice optic for the money. It's roughly $200, comes with multiple bases for different height mounting options and has one of the best warranties out there. The only negative, if you consider it a negative, would be the battery life which is listed as 300-5000 hrs...