For sundews in general, I'd go with high humidity, but cape sundews are possibly the toughest carnivorous plant of all. They do OK even here in Southern California in the dry summer air, provided the soil stays moist. My advice would be moderate to high humidity.

By the way, my D. capensis seem to love the cold, rainy, windy weather we've had here lately.

I grow my plants out in the open under low humidity but keep their media very moist, basically wet. That keep in check the spider mites so there are very few, if I ever see one. Some strands now and then but that's rare and most of the time they end up as lunch for my plants, either a sundew or a butterwort.

I wouldn't worry about spider mites. If you grow them inside your room, I doubt they will multiply into huge numbers in your quite cooler room.