First paragraph - this one is given blue border before we convert the nodelist to an array.
Second paragraph - this one should get a red border, it gets run through a (comparably slow) conversion function.
Third paragraph - this one gets a purple border. It runs a hybrid function that first tries native code and if that doesn't work ir runs the slower method.
Fourth paragraph - this one gets a green border. It uses only native code, which works in all browsers except IE. (Up to version 8, at least.) In Internet Explorer it throws an error that it can't run Array.prototype.slice.call(nodes) because a DOM NodeList is not a JavaScript object.
Conclusion: If you can just deal with it as a nodelist, that will always work. If you need an array, purple is your best bet.