I say get it!! Gopro is awesome. I've had the hero 2, hero 3+black, the hero 4 silver, and now I'm on my second hero 4 silver. When you do extreme stuff with a camera it's easy to lose it. My first one is in the bottom of a lake, the second washed away down a river, and the third is somewhere on a random mountainside in the San Gabriel mountains. lol. I've never tried the session, but I will say that I really enjoy having the screen on my 4 silver. Also, If you're diving with it you might want to consider the chest or head mount so that the camera will be attached to you and you won't lose it. Because I can tell you from my experience it's a real bummer to lose a camera that's all loaded with awesome footage.
I can just imagine, I can't really afford to lose one that is why I'm still on the fence about it. The Session is a lot smaller so it might not get caught on stuff so easily. There are so many variables to consider.. Thanks for the feedback :)