Hi msquad-
The 3.5 will be quieter than the '4. By "complete system" do you mean you have the head pipes too?
If not my suggestion would be the 3.5 slip-ons and a VIED to richen your A/F ratio from Nightrider
http://sales.nightrider.com/vied.html
This would be the most economical solution, if you like a louder sound, go for the '4 slip-ons.