I can't speak for the M&P15 Sport but have owned two M&P15s in the past (basic M4 style with carry handle) and they were both good rifles. I never had the chance to really run them through their paces as I fell on hard times and had to sell them. I waited another year and a half before buying my 3rd AR which I still have and that's the Colt 6920. I wanted one of those originally but back in 2008 when I bought the M&P15, the Colt was going for 1600-1700 dollars. Fast forward 4 years and they can be had for right around 1,100.
Many people will tell you that your choice of AR will depend on your use for it. For me personally I just wanted a good quality defensive rifle that I could bet my life on if needed. I like everything about the M4 carbine and I'm not much of a precision shooter. I'm not out there trying to make dime sized groups at 500-1,000 yards. I like to shoot no further than 300 yards. If I need to go out further than that I'll opt for a good bolt action rifle with a good scope.
The forward assist I don't think is necessary for the average shooter but I'd rather have and and not need it. It's more of a personal preference thing. Same goes for the dust cover.
There's a whole debate about what is better DI or piston. I like to keep things simple so DI works for me just fine. There's less moving parts that could break rendering the gun useless.
I hear good things about the M&P15 Sport and I may even pick one up someday.