Botox applications of the masseters are to treat square jaws resulting from clenching and grinding of the muscle.
Buccal fat removal on the other hand is to reduce the fatty bulge in the lower front of the cheeks. This bulge deepens the smiling fold and typically adds to the jowly appearance of the cheeks, hence some patients seek this treatment.
So, depending of the problem, one would need to have the appropriate method/s to achieve a slimmer face and jawline and requires a consultation with your doctor. Also, Botox does not have downtime whereas removal of buccal fat has a downtime after the surgery.
Hope this helps!