A PhD in the US includes a 'Masters' which is why the training appears more rigorous. People applying for PhD programs in Europe have already been through a Masters program. It is a common belief that the US PhD program is more rigorous than a European one.
Quality can be difficult to measure.
The number of oppurtunities in the states are higher, to that I agree. For a candidate faced between choosing a position in Europe and the states, this may not be a relevant detail.