Resolution choices, always a good debate. While your question uses specific iOS device dimensions, you should embrace the reality that DPS 2015 is about device classes more than OS/platforms. Think phone, tablet, web viewer more going forward.
Also consider the challenge of choosing a dimension for a design versus using responsive HTML for greater flexibility across devices. HTML and CSS can adjust the presentation of an article to fill any device screen, no letter/pillar boxing. The article is smooth scrolling, like a web page -- this might be a great way of building content, and you don't have to choose dimensions. Of course this means you'd use an HTML editor/system or a CMS like AEM, Drupal, or Wordpress to build your content -- definitely a different workflow. Chances are, this is where we're headed more and more.
Depending on your readership, it may be true that iPad is a good dimension for your tablet content. I think the choice of a phone dimension is slightly harder b/c it may be that you have a more diverse device pool for your phone readership. I'd likely work at the iPhone 5 size rather than larger 6 and 6+ -- that way it's more likely that a page will look good on the smaller screen and be somewhat enlarged on the larger screen. I'm more comfortable with that rather than trying to read shrunken pages on a smaller screen. (And then consider the rumors of upcoming iPhones with smaller screens... which way should you go?)
HTH
