What is the significant difference between a round bottom and a flat bottom boiling flask?

I can't decide whether or not I want a round bottom or flatbottom boiling flask. I like the idea of a flat bottom because, well, it sits. But I feel like round bottoms would be more efficient. What is the significant, rather than the obvious, difference between the two and why would one favor one over the other? Basically, why are there two types?