Maintainer here (and fan of ETV)! Currently, the most fundamental difference between Tunarr and ETV is scheduling methodology, though this is on the road to change. ETV takes a top-down approach; you schedule lists/collections (either custom, or just a whole series, for instance), clump those choices together, and customize how they "play out" their contents, e.g. "play 2 programs from series A, then play 4 programs at random from collection X".

Tunarr is, currently (and like it's predecessor dizqueTV), a bottom-up scheduler. You create a schedule with a flat list of programs (episodes, movies, etc). Then, you apply transformations to that list (grouping, padding, etc) in order to build your schedule. This is a bit of a simplification.

Both programs have a range of tedium in their scheduling, depending on how particular you are about your schedule.

Of course, ETV is more mature than Tunarr, so there are a lot of other features it has that Tunarr does not. It is also, likely, more stable. However Tunarr's streaming stability has come a long way and was the primary focus in the beginning of the project.