The only SP hub I've had any experience with is the switchable one that was taken off the market, so I don't really have anything of relevance to say about either hub that you're looking at. Speaking more generally to the experience of using dynamo hubs in cold weather, I lived in Boston for a few years and used a Shimano DH-3N80 and a Schmidt. Both were completely reliable in every temperature and weather condition. I've used both hubs for about eleven years now and briefly had an issue with the Shimano where the cone nuts were loose and the hub stopped delivering power to the contacts. I retightened the cone nuts and haven't had any issues since. The Schmidt has been a set-it-and-forget-it proposition from the start, as has a second Schmidt hub that I purchased a few years ago.