My experience is the small the quieter the sound but bigger the deeper and louder it becomes. Kind of like what a megaphone does.

Personally I've not found a reason to upgrade as of yet from my stock 3" straight pipe.

 

I have the diamond eye 5in. Straight pipe of coarse. It sounds great but the drone is pretty bad at about 2000 rpm. Pretty simple to install. Pretty cheap too.

 

I have 4" custom right off the turbo. I like the sound of the 4 better than the 5. Just depends what ya like. Dont think 5 would hurt ya if you like the sound better than the 4.

 

Is it there that much difference in sound between the two? I want all turbo..