Universal mufflers can be hard to choose from. To make it simple though, there are two components of a muffler that make a difference.
Firstly, the internal components which are generally referred to as Tri-Flow or Straight Through. The difference between the two is the amount of muffling one configuration can do against the over. The Tri-Flow muffler is the quieter option of the two. Generally, all sports exhaust use a straight through design to give your car a nice burble and note.
Secondly, the size of a muffler is also important. In essence the bigger the muffler the more quieter your car is going to sound. Sizing of mufflers works as follows (length x width & pipe diameter).