The newer player has improvements related to subtitles, especially the ability to adjust offsets. It has better seeking & thumbnails, and audio stream selection during playback. I don’t use the DVR but I think it also supports (or improves) playback of live TV.
In the past I think they’ve said the plan was to drop support for the old player. I wonder if they can support Spatial Audio in the new player or if this will change those plans.