Which one to buy
Buy the Pro if the streamer feeds an amp or a system with its own DAC, or if this is a budget multiroom setup: same local push Home Assistant integration, same AirPlay 2, Chromecast, DLNA and Roon Ready inputs.
Buy the Pro Plus when it's driving a good stereo directly and the DAC matters; its premium AKM DAC is the whole difference, and it needs no vendor login to be controlled from Home Assistant.
The Pro Plus still needs the WiiM Home app once for setup, and the Pro's companion app leans on the cloud for some features.
I haven't run any of these; the picks are based on the spec sheets and what other Home Assistant users report.