Why is the cost nearly double for USM and is it work that cost?
To sum it up: USM is fast and STM is smooth.
In the question, you're asking what is best for wildlife and sports so USM would be the best for you.
Why? USM focus designs are designed to be quick first while also keeping as quiet as possible. They're generally what is used with L series lenses (Canons professional lens range). As for STM, these aren't as fast as USM, however, they're generally quieter and smoother.
If your goal is to take photos of quick subjects then USM is the best option. However, as you pointed out these also tend to be more expensive but they also offer the fastest focus.