Well, these extensions are situated beneath the text ads as a ‘header and a list of attributes’. When these appear with your ad, they can show up to 2 headers on a computer at a time. Against this, the ads on the tablets and mobiles show only a single header. Google AdWords would automatically decide the best header or a combo of the headers to display.
Well, if you obey Google’s words, callouts are used to highlight the elements that make your products and services exclusive, while structured snippets highlight only a specific attribute of the services or products you offer. Additionally, you need to know that you could use both of these together, but with the caution that you do not duplicate the highlighted content.