There are no grammatical mistakes to use both. But in normal conversation, it sounds nicer to use "내가 배고파서" rather than use "내가 배고프니까". Because if you use "내가 배고프니까", sometimes it can be misunderstood by your friend that you don't care whether they want to eat or not. On the contrary, if you use "내가 배고파서", then it means you want to eat because you are hungry, but you are still giving them room to say how they feel about it.