What is the difference between the two declarations and will one (or has one) been discontinued?
<div data-dojo-type="dijit.Dialog" data-dojo-props="title:'Confirm Deletion'">
...
<div dojoType="dijit.Dialog" title="Confirm Deletion">
data- is an an HTML5 notation. Technically speaking <div dojoType="dijit.Dialog" title="Confirm Deletion"> is invalid HTML, because the HTML spec does not define the attributes dojoType and other attributes that are properties on widgets. The result is that when you open files in an html editor, the html editor will highlight the markup as invalid.
The HTML5 spec recognized the use of custom attributes and added data- attributes. HTML5 editors will accept any attribute begininnng with data- as valid.
Going forward, you should use the HTML5 notation.
<div data-dojo-type="dijit.Dialog" data-dojo-props="title:'Confirm Deletion'">