what is the diference between a resonator and a cat? pros.. cons? thanks for the help

 

A cat is used to filter exhaust gases. It is an emissions part.
Pros- you save the air
Cons- more than pros

A resonator is a type of muffler.
Pros- usually cheaper than a traditional muffler
Cons- may no like the sound compared to a muffler?

 

Cats also create backpressure, which engines need a certain amount of in order to run properly.

Resonators do nothing more than muffle the exhaust before it gets to the mufflers, but can be taken out will no ill side effect other than a "louder" exhaust note.