If it's the full dual CT exhaust, it's a little quieter than the Alienhead, peak hp difference is very small. Power curve is slightly different, on paper.
Looks and sound are the real difference.
If you drag race, and want every ounce of power, get the Alienhead.
If you street ride, like the dual look, and want a little less noise, get the CT.
I have seen, heard, and ridden several of both on a gen2, and both are good.