Which will hurt my carpal tunnel more: smartphone or ipad keypad?

Not sure. It depends more on the provocative posture you are assuming while using these more than the device itself.
It depends. It depends which position your wrist is in while you are using the device. The best position is with the wrist in a neutral position. Bending the wrist in extreme directions either forwards or backwards decreases the size of the carpal cannal and will aggrevate your symptoms. The position of the wrist is more important than the type of device you use.