It was a year ago when the old "80s and more" KSTJ became "The new Star". They switched exactly when SUNNY started playing Christmas music. This didn't help them at all, KSNE won the Fall book by a large margin, especially the third leg when they were all-Christmas. The dfference is 102.7 didn't promote themselves as a non-Christmas alternative last year,, there were no e-mails and I don't believe they mentioned it on the air.
Still, I think the Christmas music thing is so huge for ratings that I believe there is more available share being the second Christmas station than being the AC that doesn't play Christmas. Plus a second station could be a spoiler and shave away some of the share. But since KSNE has been doing this for so many years I believe they could still have double digits for the month even with a Christmas competitor.
What I have noticed through the years is that in December KSNE steals from a wide variety of stations and formats. Country usually has a weak December and last year even KLUC had an unusually low December. It seems strange to think a Soft AC could steal audiene from a Rhythmic CHR, but that's the power of the WIDE appeal (despite the fact that some people hate it) of all-Christmas (or all-holiday to be politically correct).