SBR's are well worth your investment and time if you enjoy shooting, it is a ball to walk up to the line and start blasting away and the folks that have an interest in unique stuff come up and start asking questions, you get to make some new friends real quick. Remember, when you shorten the barrel from say a 16 or 20 on an AR to 10.5 or 11.5 inches, it will take away accuracy on the other end. SBR's have a place and purpose, but they are fun to shoot. Even an 7.5 Kitty Kat upper has its place, but man, that is one way to make an AR a 200 yd gun, or less. I wouldn't know about AK's, were not allowed to play with things like that in CT.
Oh, don't order an upper if you have a lower ready to go unless you have your paperwork signed off and stamp in hand. ATF has been known to hammer people for, what's the term, some crap with intent because you have the upper and the means to mate it to a lower, there are people sitting in 64 sq. ft. for a couple years for doing this.