This is indeed a question worth pondering. However, I personally believe that this question should have been placed some years ago. In terms of the current situation, mainland songs have already been comparable to RTHK songs, and have even made it to the "next level" point.
In the 1980s and 1990s, Hong Kong had more contact with the outside world, and it was easier for them to open up their country and absorb a lot of foreign culture. Their songs not only contained Chinese elements, but also many foreign pop elements. This makes their songs unique and stand out in a crowd of songs that are full of Chinese flavor.
After all, people are curious. For those who are foreign, they have not touched the fresh things. People generally show great enthusiasm to accept, to pay attention to, to embrace them.
So, those RTHK songs which are fully borrowed from foreign pop songs. They are also more likely to be welcomed by people.
But now, both those RTHK artists and these mainland artists. The resources they have access to are all similar, and the level of creativity is also not similar. So, this question is not really valid when put in the context of now, in my personal opinion.
Because many people nowadays also love to listen to songs from the mainland, and not like in the 1980s and 1990s, when they enthusiastically chased after RTHK stars and sang RTHK songs.
Because, indeed, in the 1980s and 1990s, they had better access to the world and learned to sing more popular music. Many of their bands were also able to pass on more great classics. For example, the band beyond, which is still very hot and popular in melody until today, has never gone out of fashion. They were all really talented, and their songs are still very catchy even today.
But now, alone. Mainland songs are not inferior to RTHK songs either. Everyone has long since come to the same level.