I have never played FD and it is very unlikely I will. I heard is a great game, but I dislike RPGM strongly. Besides, I heard that it has quite a lot of grind and that it is very hard to access to a lot of content unless you follow a guide steps.
This game was created for those like me that did not wanted the other elements of an RPGM game but wanted the story. So originally it was going to be a simplified version of FD. However, developer ended up changing things and even characters. Nova (the super cute poster girl of the game) does not exist in FD, while other characters do not exist here.
MY ADVICE, if you are like me, which only cares about the story, play this one. If after you finish chapter 4 you still want more and cannot wait, you could try FD and see what happens.