Everyones ears are different, mine are pretty sensitive to drone. With a 350hp straight-thru center muffler the 6db will give some interior resonance that I can hear but it's not bothersome to me. IMO the 4db on a 350hp center muffler has no resonance. If I owned one and had to drive it as a daily driver "I" would probably go with a 4db myself (@45 years of age). If it were just a weekend car then maybe an 11db eliminator kit.
So it would depend on who is in the car with you when you drive, ie wife, kids, inlaws in the back seat. How much driving you do. Type of driving. How loud you listen to your music. How old you are. A few of the things I would normally ask when recommending an exhaust.