Would fillers to define or masseter slimming be better for my long/flat face and large jaw? (Photos)

Hey there - thanks for the question!
I think you would get a good result from masseter botox injections. They help with slimming down of a more square jaw line, as well as the bonus of stopping tooth grinding at night.
Results will usually start to appear within about 2 months whereas tooth grinding will take 7 days to settle down.

Injecting a muscle relaxant in the masseters helps with facial slimming. It will relax the muscle and with time the size of the muscle will decrease.
This decreases the width of the jaw. Patients like this because they feel the face looks more feminine  and smaller
Assesments are best made face to face with detailed consultation to work out the best solution for you.