javafx.scene.layout.Priority should implement Comparable to make it easier to work with multiple nodes that have different Priorities. This would allow the following code to work:
Sequences.min(for (child in children) XContainer.getNodeHGrow(child));

Here is a sample PriorityComparator that satisfies this requirement, and may be useful as a default implementation:
public class PriorityComparator implements Comparator<Priority> {
    @Override
    public int compare(Priority p1, Priority p2) {
        int p1int = convertToInt(p1);
        int p2int = convertToInt(p2);
        return p1int - p2int;
    }
    private int convertToInt(Priority p) {
        return p == Priority.ALWAYS ? 2 :
            p == Priority.SOMETIMES ? 1 : 0;
    }
}

And of course a test case (written in JavaFX using JFXtras Test):
public class PriorityComparatorTest extends Test {}

public function run() {
    perform("PriorityComparator should find", [
        for (p in Priority.values()) {
            Test {
                say: "{p} == {p}"
                do: function() {PriorityComparator {}.compare(p, p)}
                expect: equalTo(0)
            }
        }
        for (p in Priority.values() where p != Priority.NEVER) {
            Test {
                say: "NEVER < {p}"
                do: function() {PriorityComparator {}.compare(Priority.NEVER, p)}
                expect: lessThan(0)
            }
        }
        for (p in Priority.values() where p != Priority.ALWAYS) {
            Test {
                say: "ALWAYS > {p}"
                do: function() {PriorityComparator {}.compare(Priority.ALWAYS, p)}
                expect: greaterThan(0)
            }
        }
    ]);
}