Increasing Masseter Muscle Size: which is better?

TLDR: Hard gum (mastic, falim etc) VS the rubber exercise pieces. What is best for Masseter increase?

And by the rubber exercise pieces, I do not mean the singular ball that you place in the middle of the mouth (I know that can be very prone to causing TMJ). I mean the individual pieces where you place one on each side of the jaw at the same time and then chew - bear in mind these pieces would be placed on the molars on each side.

Which is best? My feeling is the rubber pieces are better for symmetry, as you have them in at the same time, and just chew up and down. Whereas with gum you can have varied chewing patterns.