the 40 series will be a little louder than the 50, but it may not flow quite as well (not a big difference though). but if you get the Delta Flow 40 series (a little more expensive) it supposedly isnt as loud until WOT, doesnt have as much interior resonance, and supposedly flows much better than the Original 40 series. I was tricked by the guy at Jegs who told me the one i was buying was an Original series. Well i figured out it was not and wasnt happy at first. but then the muffler broke in. i was definitely happy with it then. and the my opinion is, the louder the better. so while the original series may be a little louder, the Delta Flows are also a great muffler. if i were you, i'd go with a 40 series Delta Flow.
Jegs
Original 40 series $59.99
Delta Flow 40 series $72.99 <--------- Go with this one
50 series $79.99 i believe
Jegs
Original 40 series $59.99
Delta Flow 40 series $72.99 <--------- Go with this one
50 series $79.99 i believe