I don’t have access to a RT4k but the main advantages of the RT4k at the moment are, as far as I’ve seen (other than 4k output of course)
Composite and S-video inputs
Automatic picture presets (applies optimal timings based on signal detected)
Image rotation
Extensive colour correction for HDR mode etc
The RT4k has some disadvantages over the OSSC Pro too, namely
No expansion port (so no analogue output card for instance)
No USB port (so no external device control, such as for external RGB switches)
No line multiplier modes (though to be fair it’s very low latency anyway and you probably wouldn’t notice the difference)
You should also consider the PixelFX Morph which is coming in the new year (I have a prototype and it’s great so far). You can use this with your existing scalers or purchase the analogue bridge add-on for a more integrated solution. It’s also $250 cheaper than the RetroTINK 4k (including the analogue bridge add on). Currently it lacks picture rotation but should have most of the RT4ks other features on launch.