Assuming it is unrealistic to carry two spare wheels with you at all times, we would suggest fitting your spare with the same winter tyre as the rest of your vehicle. In the event of a puncture you could then replace with your spare and take it to a tyre fitter to have it replaced immediately if had to be fitted in the wrong direction - we would suggest this is a more desirable outcome than fitting a conventional tyre to your spare, as it may be more difficult to source a matching winter tyre from a tyre fitter in the event of a puncture.