As others have said, the SSI's into a 2-inlet muffler deletes the catalytic converter, as it is a stainless steel replica of the pre-75 (pre emissions) exhaust. Here is a pic of mine to demonstrate that there are no cats.
As for myself, I have a Dansk 2in/1out muffler factory replacement muffler, and highly recommend it. Secondly, I'd have an M&K 2in/1out as it is lighter than the Dansk and doesn't have the drone issues associated with dual outlet mufflers.
The exhaust upgrade is incredible. Not only does it sound incredible, but it looks fantastic (for those nerdy enough to look), the mid-range torque is vastly improved, the top end horsepower bump is great, and the improvement in cabin heat is much appreciated.