I’m planning to migrate from RPi3 to an NUC. I see the recommended way to get the HA OS onto a NUC is to load a trial copy of Ubuntu onto a USB, fire that up, then copy the HA OS onto the NUC SSD. However, I see that once I have Ubuntu running, I can install HA as an app.
Would anyone chime in on the merits of one over the other? Thanks!
What exactly do you mean “as an app”?
Because I don’t see a reference to “app” in the “recommended way” which simply offers two methods of writing the HA OS image to the disk.
He means using the “live boot” method - using a USB based install of the Linux OS and then writing HAOS to the NUC SSD.
If you install via the live boot method - then you will have HAOS running on the NUC.
If you run Ubuntu on the NUC and want HA running as well you are limited to Home Assistant Core and Home Assistant Container methods - both of which require a bit of fiddling and are definitely not user friendly.
If you insist on running Ubuntu I would recommend running HAOS in a VM.
All the above methods are described in the docs under the page you linked or under Linux installs.
My personal preference - if you have the hardware spare is to dedicate the NUC to HA using HAOS - more convenient, better performance and simpler to install, backup and restore.
If by “app” you mean as a snap installation, that particular method installs Home Assistant Core only and is maintained by a third-party (i.e. it’s an unofficial method of installing HA Core).
If using a NUC might I suggest using Proxmox it easier than falling of a ladder 
If you install Ubuntu, HA is listed as an app you can install (it’s not discussed in the link.) As zoogara comments below, I thinkI like the HA OS option better.
Did you read my previous post regarding the unsupported snap installation?
There’s no officially supported “Ubuntu app” version of Home Assistant.
I’ll just add that I run my HA on a NUC as a VM. I use Ubuntu 22.04 Desktop as the Host OS and I use QEMU/KVM/Virtual-Manager as the VM. I can do a remote desktop to the NUC which gives me access to the Ubuntu Desktop, and it can also give me access to Virtual Manager which in turn can provide me with access to the HassOS console if I ever need it. I will caution that it is not so easy to setup, but there is community guide for setting this up at least on an Ubuntu Server (as opposed to Ubuntu Desktop).