Most of the information circulating on the internet is from people regurgitating information from unreliable sources.
The only way to conclusive determine whether the Sigma chip is better that the Realtek chip is to carry out a high definition HDMI A B test on a digital TV calibrated using professional calibration software.
The high definition video source material would have to be a virgin rip bluray containing vibrant colours varying from bright to dark scenes.
Be aware that all movies are colour and lighting corrected to the locality they are filmed. Film will vary from digital Red Camera.
Realistically purchase the device that suits your viewing lifestyle because you can put two virgin identical digital TV’s side by side and both will, out of the box, display a different colour. That is why digital TV manufacturers provide colour controls.