-기 때문에 states a simple reason for something as a fact, like "because", "as", or "since". -ㄹ 테니까 mentions a new plan or expected situation that might affect the future course of action. So -기 때문에 usually makes a static expression, while -ㄹ 테니까 is more dynamic and fluid in meaning. For example, 1. 비가 왔기 때문에 날씨가 시원해졌다 = Because we had rain, it feels cooler now. 2. 그 사람은 경험이 많기 때문에 잘할 거예요 = As he has a lot of experience, I believe he'll do well. 3. 필요하면 나도 도울 테니까 열심히 해 봐 = I will help you too if necessary, so please try your best. 4. 영철이가 지금쯤 부산에 도챡했을 테니까 전화 해 보자 = Let's call 영철 as he must have arrived in Busan by now.