Does your pulse rate increase more in sit-ups or push ups?

Better for what? That is like saying "which is better a car or a boat". They are two toatally different exercises that focus on different parts of the body.

Sit ups are primarily a stregth/endurence exercise of mainly the abdominals (esp. the rectus abdominis)

Push ups are a stregth/endurence exercise focusing on the upper body (esp. the chest, shoulders and triceps)