I have had both on different cars, my earlier car having the single sided and you are fully aware of the nice rumble and occasional pop but quite a muted experience, a 3.9 as well.
My present car came with the Librands Sport dual system but standard cast iron manifold, I can't say I have noticed any difference in performance but the sound is full on rumble with lots of popping on over run at around 1,500rpm. Ground clearance not an issue.
It suits me perfectly being a dual hearing aid old boy and haven't found it a problem on longer runs but can see that if your hearing is good it could become intrusive.
I would advise meeting up with someone that has the system you fancy and give it a listen as sound is such a subjective thing.
Good luck with your choice and good to hear you have a Plus 8 again to enjoy.
