Question

Well, the question is in the title. Is there any difference (performance, caveats) between multiple selector

jQuery("selector1, selector2")

and adding elements to selection with add:

jQuery("selector1").add("selector2")

Answer 1

Yes, the first will create a jQuery object that contains elements matched by both selectors. The second will create an object that has elements matched by the first selector, then create and return a new object with both, without modifying the first object.

For example:

var jq1 = $('h1, h2'); // Will contain all <h1> and <h2> elements.

jq1.add('h3');
alert(jq1.filter('h3').length); // Will alert 0, because the 
                                // original object was not modified.

jq1 = jq1.add('h3');
alert(jq1.filter('h3').length); // Will alert the number of <h3> elements.