Well, my dear Watson, this is a very good question! It does sound at first like a great idea to protect your toothbrush from the exterior with either. But the answer is neither. Why? Well, for a very
simple reason... bacteria, which already multiply at a vertiginous rate on a toothbrush, love it even more when it remains wet and warm; two characteristics helped by both toothbrush holders and covers. You can nonetheless definitely use either one when you need to move your toothbrush from the bathroom, on a trip for example. But do make sure that your toothbrush is fully dry when you place it in it.