You would use .attr(), as .prop() is more commonly used for boolean properties such as checked, selected, etc – though it is certainly possible with .prop it’s arguably less clear as per your intent
Though I do believe that ultimately they are very similar (or used to be) functionality-wise
Just a note: the jQuery API site seems to follow the boolean ‘sway’:
.prop() – Examples use checked and disabled
.attr() – Examples use alt title and src