My experience riding both a 700c and a 26" in the winter with studded tires was that the 700c was much better at rolling over the constant small bumps in winter riding. It cut through snow mix to the surface better, it was faster, it was a more comfortable ride.
My suggestion would be to sell the 700c and buy a new more comfortable 700c bike.
That being said a super stiff winter bike rather sucks for winter riding. If I **had** to choose I might still go with the 26" because the drawbacks are worth the improvement in ride quality. But...like I said the choice I actually made is getting rid of the overly stiff bike and buying something new that isn't overly stiff to replace it.