both are fine but mean slightly different things. I wonder how - you are wondering how he is going to decide or choose. What process or method, you don't care about his actual decision. You care how. I wonder what - you don't care how he decides or what he does to help him decide. You just care about the outcome, You care about his decision. You care what. Hope this helps, Rob.

I wonder how you'll decide: which tools to reach the decision I wonder what you'll decide: which will be the decision