One of the good things about the AR is its modularity.
Pick up the Sport. Get good with it. If you need/want a different stock, no problem. I prefer the A2 length, both of my AR's use rifle-length tubes and stocks. Changing the parts out is not difficult at all. Same for the handguard, barrels, trigger, etc. If you have a modicum of skill and some basic tools, the options are nearly endless.
Good equipment will never be a substitute for good training.