The 15 gem thing will be up for a while until Episode releases the actual gem prices (that authors have personally set) to Android users.
I personally prefer to have the supporting the author or buying the points because then I know what I can expect from either choice (for support, I am simply supporting the author. And points is so that the scenes or characters I want to see/be with or have a good relationship with will come up.)
What a reader views as a good bonus scene and what an author views a good bonus scene will always be different and if the readers don’t like the scene, they might regret spending the gems. Providing support the author/point choices provide low risk and are the longest running type of gem choices community authors have offered. So I’d recommend providing those.