IMO they’re currently about as good, but they still serve slightly different purposes; Immich doesn’t fully support existing libraries because they don’t support nested albums, and PhotoPrism doesn’t have a native official app and therefore only supports sync through 3rd party apps.

One thing that is relevant to mention is that Immich uses OpenStreetMap in a way that isn’t kosher, and while they’re receptive to changing that, it clearly isn’t a priority to them. It doesn’t reflect too positively on them that they’d essentially abuse another (far more important) FLOSS project like that, while PhotoPrism has made it a priority to avoid such issues.