If someone asked me to name essential apps for the their iPhone, iPod or iPad, one of these two would be in my reply.
If you like to read longer items on the internet, or if you like to save items to read later, and especially if you like to read web articles on your phone or iPad, one of these two applications is vital for you. They work in browsers like Internet Explorer, Safari or Firefox; on Apple iOS devices; and on Android devices.
You don’t need both. Their functions are nearly identical. Both are free to sign up and to read in a browser, but charge for their mobile apps. Instapaper is $4.99 and the same app works on iPad, iPhone and iPod. Read It Later is $2.99 and it too works on all three devices. Read It Later has a free app with less functionality.
Instapaper seems the more popular of the two. Read It Later has a prettier presentation, at least on the iPad.