Living in the north, I personally recommend a mat over a lamp. In the winter it can get very dry with the heat running all the time, and having an overhead lamp heating the air in the enclosure can exacerbate that. Lamps are perfectly fine though if you decide to use one, and you shouldn't have too much trouble with humidity through the summer at least. Just be prepared to do some misting or provide a wet hide during colder months to prevent stuck sheds.