It is all about configuration and how you are going to use the rifle. Use always should drive configuration! My advice is buy the complete rifle that matches the mental image of your perfect rifle right out of the box or built your perfect rifle. For newbies I always recommend complete rifles or at least complete uppers and complete lowers which just need to be pinned together. Builds come later.... LOL
The Sport II is a basic no frills M4 A2 clone. It has a a 1/9 twist barrel which will not always stabilize high weight rounds like 69gr. It has a A2 post so if you want a free float rail this is not for you unless you want to take of the post replace the gas block and install the new rail. These used to sell for $499 at a few places but now have moved up to $549-$600+. This is a basic budget rifle if you like the configuration and don't want to change it it is a good value. If you are going to swap things out there are better starting platforms IMHO.
Product: Model M&P15 Sport™
The 15X is little bit of a step up. It comes with a Troy or UTG quad rail, which IMHO are outdated, Troy rear battle sight and a 1/9 twist barrel. Barrel profile is also different it is an M4 mid weight profile. Everything else is basically the same as the Sport II. Unless you are getting a smoking deal on these I would pass. You are better off with the sport and replacing the rail, gas block and sights based on the pricing I have seen in the past. There are better setups for the money.
Product: Model M&P15X Rifle
The Tactical is the real upgrade from the Sport not a sort of upgrade. You get a 1/8 twist barrel which I prefer. You get the similar rail Troy or UTG rail but the A2 front post is gone and your rail is free floating. You go backwards on the sights and get Magpul MBUS. For me the cost of these rifles pushes me in other directions but if you like the configuration and the price is right it is a quality rifle and better than the Sport II in terms of spec. You have to choose if the upgrades are worth it.
Product: Model M&P15T - 5.56 mm, Tactical
Again think about how you plan to use the rifle and let it drive the configuration. Is is a pure range plinker. It is a home defense rifle? Is it a truck gun? 3 gun or other sort of competition gun? Is it combination of multiple rolls?