I am a firm believer in winter tires due to the traction they get when the bad weather hits. The traction between all-seasons and winter tires is night and day to me when there is white stuff on the ground.
I usually run my winter tires on my truck from November to April.
The only reason I would go with all-seasons was if I didn't want to spend the extra dough on a second set of wheels and I didn't want to take advantage of the Boxster abilities at autocrosses. I'm not even sure they make winter tires that would fit your wheels as 18" may be a little bit big, and then you add the profile and the width?
If I were in your position (my Boxster is going to be parked during the winter) I would buy the second set of wheels in 17" size (narrow) and throw on some Blizzaks. I would then keep performance tires on my 18"s for warm weather driving and autocrosses. Of course you could throw in a 3rd set of wheels for competition tires...
Just my two cents...