Form action according to radio button selection does not work

I am doing a validation of 2 radios buttons, where depending on which one the selected form forwards to a different page. I'm using Jquery, as below:

jQuery(document).ready(function($) {
    var form = $('form[name="cadastro"]'),
        radio = $('input[name="MERCADO"]'),
        choice = '';

    radio.change(function(e) {
        choice = this.value;
        if (choice === "ME") {
           form.attr('action', 'Checklist-P1-ME.php');
        } else {
           form.attr('action', 'Checklist-P1.php');
        }
    });
});

The radios are these:

    <input type="radio" name="MERCADO" value="MI" CHECKED/>MERCADO INTERNO<br><br>
    <input type="radio" name="MERCADO" value="ME" />MERCADO EXTERNO<br><br><br>

But what happens is that Jquery only recognizes the MI value if I click on the ME and then click back on the MI. It's as if it does not recognize the checked predefined.

Any suggestions?