As someone who has had the Unitronic turboback (catted) and the Corsa (APR) catback with downpipe, I feel like I can give a good opinion. So my turboback Unitronic was pretty quiet. Not a lot louder than stock. But one thing that annoyed me about it (and I ultimately have taken it off the car) was quietly driving around town. Now I know people may not believe me and it seems counter intuitive, but even though that is a fairly quiet exhaust, I found it to be loud when I didnt want it to be.
Let me explain. My Car is a 6MT, and I found when just driving around locally (25-40 mph), if I gave the car any significant amount of gas under 2500 rpms, the cabin had a loud "boomy' style noise. now, that may be "drone" or not. I dont know but i hated it. I would usually end up taking the revs over 3k so on the next shift, I wouldnt hit the gas below 2k. Doing that sounded horrible. I would drive the car in higher rpms to reduce the boom.
I then replaced it with the Corsa catback. I have now run this catback with both a catless and Unitronic catted midpipe and that low end boomyness is gone. Around town it is actually quieter in the cabin while being louder outside. It is louder both in the cabin at outside while WOT and over 3500. I can also give the car a reasonable amount of gas in a higher gear and not have the boomyness inside the cabin.
im not sure if this makes any sense without hearing it in person but there is something to it when people say APR has no drone. The decibel level is louder but then note is higher pitched. Which for me, translated to less annoying interior humming/boom/drone/whatever. Take that for what its worth.
Final note: Unitronic is very quiet on the highway. If you do a lot of commuting it may be the way to go. I am currently looking into adding a resonator to quiet down the Corsa on the highway. Its loud when cruising over 70mph.