I think in reality the difference in utility ranges across many orders of magnitude, whereas funness does not change nearly as much. I used to be convinced that research outside of pure math/CS would be incredibly boring, but it turns out that work in basically any field that is trying to solve problems we don't know how to solve is quite fun. Do I think that machine learning is more interesting than synthetic biology? Yes, but not enough that if Paul convinced me that synthetic biology research was more useful at the margins I wouldn't switch.