I used Duet Display for a long time. I was frustrated by their inability to work with Apple to get their software to work like it used to work.
Duet frustrated me further by having an outdated web site, their "Check for Updates" didn't work, and the best way to get information and new software was to ping their employees on Twitter and wait for an emailed link.
Locking your screen? Duet required your password because it relies on AirPlay. So even if you have an Apple Watch correctly configured, you can't unlock your Mac with it while using Duet.
Luna seemed a second coming. Hardware? Check. Duet is all software. Wifi? Check. Duet needs USB unless you have a pro account. Resolutions? Check. Duet can't do full iPad Pro resolutions because of its software basis. Luna resolves that with their Micro-DisplayPort (or USB-C) dongle.
Luna does display fully on the iPad Pro. Wifi works, sort of, but USB is more reliable and the only option in a corporate environment. Unlocking with an Apple Watch is also a no-go.
Oh, but the resolution thing - it always defaults to the HiDPI resolution. So even if you want to always use higher resolution (which still looks great), you have to manually select it every time. When Luna needs to reconnect? You have to manually reconfigure the resolution.
There's lag even with the dongle and USB.
Trying to run my MacBook Air in headless mode with Luna Display was ultimately unsatisfactory.
One key thing Duet offers that Luna doesn't is the touch bar at the bottom of the screen. While there are many problems with it, it had its utility.
My advice: unless you need your iPad Pro to work at full resolution, you need touchbar capabilities on the iPad, or you need non iPad Pro support (neither does well with iPhones) - get Duet. Otherwise, get Luna.