which is better if any?
whats the differences and also how are they simlar?
Im trying to get the best performing muffler for my daily driver.

 

one of the differences is the lenght of the muffler. the rsx is longer than the SI. I have both and i'm waiting to put on my si one in about a week. I'll be able to get more side to side specs later.
As far as performance, can't help you there. some say that the 07 one has better flow, I have yet to find out.

 

i had a 06 si muffler on a b16 with 2.5 exhaust and cat. it sounded like CRAP. So raspy. You will have to get a big resonator to quiet it down. What type of motor are you running BTW?

 

ok so i just went from an 02 rsx-s muffler to an 07 si muffler. I'd say i like it. si muffler has a slightly better flow but not much. It's a little louder than the rsx one but it's not as loud as an aftermarket one.
By the butt dyno, i'd say small increase on torque from mid to high (~3.5-7 k)rpms but a decrease on the lower end.
Btw, my application is on a 16y8 with a 2.25 piping catback and a 24 in. resonator.