They both work fine, just try not to put them directly against your skin. If you have glove or mitten liners, you might want to put them outside the liner to protect your hands. The boot ones work well too, particularly for things like extended snowmobiling where you might not be moving around much.

One thing to pay attention to: the heaters really do expire! If you use them too far after their expiration date, they will not generate much, if any, heat and will be pretty useless. If you keep any in an emergency kit, be sure to rotate fresh stock in every year or so.