I want to know which of these two drugs is more likely to cause weight gain: sertraline or amitriptyline...

Misinformed?: Drugs do not cause weight gain. Eating more than normal causes weight gain. Both of these drugs are known to cause some patients to feel more hungry than normal. If I had to guess then Amitryptiline is more likely than sertraline. But that is not 100% valid.

More often: amitriptyline has been known to cause an increase in appetite and subsequent weight gain.