personally, I have never understood an unequal duck stance (ie 18/-9, 12/-17). To me that is straight up uncomfortable.
I ride 15/15 wide stance, and couldnt be happier. easy to ride switch and do 180's, feels balanced and stable.
I hope you mean you ride 15/-15. And yes I agree that an equal duck stance feels so much better than an unequal, but I'm sure people's legs are built differently and their knees don't bend at quite the same angle. Personally I ride 12/-12.