I use both. I also have a digital thermomater inside the box to always know the exact temperature inside. I look at it this way, Its a worthy investment to have a heated pad (a good one) If It keeps me from loosing one pup, it pays for itself several times over. Remember, Puppies cannot regulate their own body heat for the first two weeks of their life. Once a pup gets chilled.. They're usually done for.
However, You can also get them to warm. Make sure you are heating the box, but not too much. I suggest you keep your box at 85-90 Degrees the first week, 80 the 2nd, 75 the 3rd, and 70 the 4th & 5th Weeks and no less than 55-60 thereafter.
We use a fully insulated enclosed "whelping Den" so, I have never used a whelping box per say. I will warn you to keep the floor of the box insulated against the cold. The #1 reason for puppies dieing in the first 2 weeks of life is cold.
These are just my suggestions.