Avatron Software's Air Display iOS app has been around since 2010, providing a way for Mac and PC users to use an iPhone or iPad as a second screen over a Wi-Fi connection. That method of connection was fine... although there was a bit of lag due to network latency and the connection sometimes outright failed. In January of 2015, a competitor -- Duet Display (US$7.99) -- hit the scene, and immediately captured the minds and wallets of a lot of users simply because it used a fast Lightning connection rather than Wi-Fi, allowing instantaneous updates with no lag. Avatron has wasted no time bringing Air Display up to snuff with Air Display 3.0 (US$14.99), which also bakes in features formerly found in the company's Air Stylus app.
To work properly with a Mac (Air Display 3 doesn't work with Windows... yet), you need to first install the free driver application onto your desktop or laptop computer, and then reboot. Once that's done, plugging a Lightning cable into your iOS device enables the display connection and your iPad or iPhone (running Air Display 3.0, of course) becomes a second display for your computer -- complete with touch control.
Unlike Duet Display, you can still use the Wi-Fi connection for wireless control of your Mac if you'd like. Air Display 3.0 also includes the features of Air Stylus, which means that you can pair a pressure-sensitive stylus like the Wacom Bamboo Stylus Fineline or Adonit Jot Touch to your Mac, then use your iPad or iPhone as a graphics tablet for your Mac. I tested the Wacom Bamboo stylus as a pointing device on my Mac, and it worked beautifully. For artists or graphics designers, this is a wonderful and inexpensive way to get a pressure-sensitive graphics tablet.
I ran some streaming video to an iPad Air 2 and iPhone 6 Plus over both the Wi-Fi and Lightning connections; both worked, but there was a bit of hesitation and latency with Wi-Fi that disappeared when I was connected over Lightning.
You can actually add up to four iOS "displays" to your Mac provided you have enough free ports and Lightning cables. That's not particularly practical, but it is cool... When using your iPad or iPhone as a second screen, it's quite easy to use an on-screen keyboard for typing into a Mac app.
I should note that I was running a late beta version of Air Display 3.0 and that I had some issues, particularly when using the app with an iPhone 6 Plus. The Mac-based host app crashed a few times, taking my menu bar buttons with it. A reboot cleared that issue.
Fellow veteran tech blogger James Kendrick had a really good point about Air Display 3.0 on his iPad and MacBook Air in a personal tweet the other day, saying "It is so useful to have a 2-monitor system that fits in a tiny bag." That to me is the point behind apps like Air Display 3.0 and Duet Display.
So, what's the best of those two apps? Well, if you don't need the "pressure sensitive graphics tablet" or Wi-Fi capabilities of Air Display 3.0, the smaller price tag of Duet Display is more attractive. If having a wireless second monitor is important, then by all means spend the extra $7 on Air Display 3.0