Libreelec vs kodi + lubuntu

The kodi experience should be very similar, although I think LE has some patches to kodi.

LE is a more integrated approach, you'll never see a desktop and you boot directly to kodi.

ubuntu has the ability to install anything in the ubuntu (or ubuntu friendly) repos, open a desktop, etc, but then again with docker LE has a lot more abilities to install other software too.

Clearly we are all here because we like LE. Therefore I may be biased, but I prefer LE for a purely kodi machine.

Ubuntu will be more flexible for installing random apps, but flexibility also brings complexity for you to manage. LibreELEC is less flexible (a limited range of apps available from our add-on repo) but hard to break (most of the filesystem is read-only) and easier to update. If you don't have much experience with Linux, LibreELEC should be easier for you. Kodi should be functionally the same in either distro.

NB: You can install LE to a USB key (boot from the installer USB, install to a second USB) for testing.