X or H?

[91LX2Bfast]

97snakedriver is right on. The reason you heard of a light coming on in other 4.6L stangs is because they are running the offroad H-pipe (w/o cats). And thats where also you are worried about it being too loud. The offroad is a lot louder (sounds mean in my opinion) then stock, and the catalytic H-pipe will sound slightly louder, but nothing that will get you in trouble.
The offroad H-pipe seems cheaper, but by the time you buy the MIL eliminators, it will be almost as much as a high flow (catalytic) H-pipe. The offroad pipe is also illegal in most states, so you will have to swap pipes come emissions time. My advice is to get a High Flow catalytic H-pipe, and enjoy the power and sound the legal way.