From listening to both running side by side the Stromung is louder than the Cobb. I have a twin tip Stromung full 3". The sound I don't think you can beat. It is very quiet though. All the "quiet" mufflers are pretty close. Hush, Whisper, Stromung, Cobb, Prodrive, EVO2.
Flow is very subjective it's going to be hard to say which one flows better. Could be the downpipe design restricing things etc. etc. but you know that.