Cole
As ur driver improves, his training gains get smaller, i.e. he gets harder to be trained. Physical has a smaller impact on driver level. That means, training physical first is less damage for the trainings later on.
If need a good performing driver right now, training driving ability might the way to go.
If you think long term, training physical first is a must!
Also, as stamina increases, health loss during race will be smaller. So you will have more health after race so you can train your driver more