When planning a purchase for an older piece of technology, it's important to have comparable specs in an easy to read format (like a table).
I'm going to be buying an iPad and one generation old iPhone 5 or 5s. As of this writing, the iPhone 6 and 6 Plus are the latest models and they are already a year old. Since Apple has been releasing a new iPhone each year it would be smart to wait until the iPhone 7 is released before dropping money on something that will then be two generations old instead of one.
iPhone Model comparison (Wikipedia table)
iPhone, iPad, Mac Buyer's Guide: Know When to Buy
To help time your Apple product purchases, check out the MacRumors buying guide link above. It will show estimates for when a new product generation is imminent, the average number of days between releases, and how many days it's been since the newest release.
