I often get this question from students. As a teacher that practises and teaches both subjects, I hope to offer some insights on them.
To begin with, both are independent systems but come under the same Xuan Kong umbrella. You can learn Liu Fa without knowing Flying Stars and vice versa. There is no direct connection but yet they have common technical terms. Hence, this created certain anxiety among practitioners that Flying Stars is a pre-requisite if they wish to study Liu Fa.
Let me go straight to the point on how different are they. Perhaps a table for comparison is apt.