Prone is incredibly stable, you will love it!
Seated with ankles hooked is good, but if you are flexible enough (I am not, any more) to sit cross-legged with your knees flat toward the ground, and your elbows locked into the inside corners of your bent knees, you will find that to be almost as stable. You're all triangulated and the sling is pulling the rifle irresistibly into your shoulder.
I used the web sling for years because the leather sling seemed too complicated. Once I was shown how to use it, it is better and rock solid.