Is it harder to get a helicopter or an airplane license?

Helicopter definitely.

Helicopters are much more expensive to rent for lessons (assuming you can even find one for rent in your area) and burn more fuel per hour.

The act of flying a helicopter is just more demanding mentally.

Most helicopter pilots start with the fixed-wing license first and then move on to helicopters.

ANS 2 - At one of my postings in the RAF, we trained helicopter pilots for the RAF and Royal Navy. -They all had to start having passed the Fixed Wing Course before they were allowed to train on helicopters, and even so, a number of them always flunked the course.