Both systems use different methods and have different options to estimate your eating level, based on estimate of calories to be burned by end of day plus what you have burned already.

MFP will keep getting corrected by Fitbit data if you sync frequently.
But really, after a couple days, you should have a decent total daily figure to plan the meals earlier in day, and adjust the evening stuff to whatever happened.

Use Fitbit for activity related things - manually logging workouts best done manually or not captured automatically.

Use MFP for eating related things - goals and logging food.

What is mfp