Some things to note based on my usage:
1. Lightning's USB 3.0 speeds exist only when using the Lightning to USB 3.0 adapter, and even then for certain usages only. Unfortunately, direct transfers to/from a computer is not one of them; I can't even connect to a computer with that adapter directly. If using a Lightning to USB cable, I can hit up to only 30 MB/s when using iFunBox for transferring files to and from a computer.
2. Airplay has more or less been able to do a good job on handling 1080p 60FPS output. Only thing is you need to have a stable network connection (either WiFi or Ethernet). Even then, I am having some minor issues with occasional audio drop outs and even no audio at times (but I think this is more to do with Airplaying from an iPad Pro with iOS 11.4.1 to a tvOS 11.2.6 Apple TV?). Given these, I don't see how Airplay can scale to 4K output well
3. The Lightning to HDMI connector is actually powered by an A8 SoC, which may explain why you cannot do 4K with it. The color it gives out looks weird (somewhat more yellowish) compared to Airplay from Apple TV. Only good thing is that it is much cheaper and easier to setup.
4. The Lightning connector may prove to be more sturdy than any form of the USB connector since there is nothing inside to break, but conversely, dust has been known to enter these Lightning connectors and give connection issues due to this. Granted, it is slightly easier to clean than USB C but still, both ports are hard to clean in the first place. Hence, can't really say that one connector is better than the other for such reasons.
5. Similarly, I find the Lightning port's pins to feel vulerable too; in the long run (especially if you are planning to plug in one of the Lightning adapters for long term usage) it may become too loose to be useful. The USB C port's don't feel this vulnerable unless you somehow can break that part in the middle of the port.