Rather than
Use “rather than” to state preferences and choices. Such as:
“He wanted to be a doctor rather than a teacher”
“I prefer to eat pizza rather than salad.”
“I decided to write rather than phone.”
“In the end, we decided to go to Toronto on Friday rather than on Sunday.”
“He’s probably just lazy, rather than stupid.”
Note how each sentence states a choice or preference. There is not just a comparison.
Than
Use “than” to compare two things without making a choice. As in:
“Doctors make more money than teachers.”
“Waking up early is more beneficial than staying in bed late.”
“Writing is slower than calling.”
“Cats are smaller than dogs.”
“Attending university at home is cheaper than going to another country.”
Note how every sentence makes a comparison, but no actual choice is made.