Just do both. If you use only the rear than you risk slipping on ice while turning and vice versa. I ebay'd 2 sets of winter tires for the cx bike - new pairs of Conti Nordic Spikes and Conti Top Contact Winters...when there is ice present I throw the studs on, if it no ice then I run the Winters (un-studded but soft compound made for cold weather/occasional ice patches). I picked up both pairs for less than a brand new set of either. The best time to look, of course, is throughout the year when it is not winter.
Last year, during return of the polar vortex when we saw routine temperatures of 0 and below, with plenty of ice and snow, I rode on Conti Top Contacts (apparently I love Continentals because my car also has a set) and a few times nearly killed myself riding in the really slick stuff because I couldn't ride fast enough to build up body heat to keep me warm. It may take some initial investment, but there is absolutely no going wrong with a full set of studs for the winter.