Just so you’re aware, this is less of a theory and just a straight up fact. The brother of Vegeta and the second Prince of the Saiyan’s in Tarble is stronger than the brother of Goku in Raditz. That means that at least one of King Vegeta’s children, the most unlikely one as well, is stronger than one of Bardock’s children. Also, no, technically it doesn’t matter that Raditz is canon while Tarble is semi-canon, that doesn’t make Raditz stronger. Again, not a theory, this is just a fact and I will explain why.
When Raditz first burst into the scene in the early stages of Dragon Ball Z, he shocked us with numerous revelations. The Saiyan told us of Goku’s true mission to kill everything on Earth and the survival of other powerful aliens. Furthermore, his power seemed to monstrous at the time, surpassing the strongest two warriors on earth by a landslide. Those two warriors in Piccolo and Goku only were able to narrowly defeat Raditz but had he wanted to, Raditz could have disposed of them quite quickly.
However, as the story continued to the arrival of Vegeta and Nappa, we found out that Raditz’s power-level was only about 1,000. This is barley above the level of a Saibamen. Furthermore, every Z Fighter at the time of the Saiyan’s arrival, had long surpassed that level of power with training, which meant yes, Raditz was surpassed as well and that’s probably why we never hear from him again in any capacity.
Meanwhile, as for Tarble, he was banished from the Planet Vegeta for being a disgrace to the Saiyan race. Tarble disliked conflict and hated battle as a result. This caused him to lack fighting skill and aggression. However, that doesn’t mean that he didn’t have power within, just that he didn’t like using it. That’s pretty much Gohan in a nutshell and Gohan isn’t someone you want to piss off.
Tarble was later able to fight off, survive, save his family and even flee to Earth after clashing with two of Frieza’s former Abo and Kado. The two had also surpassed their former master Frieza at the time in terms of raw strength. For any regular Saiyan, especially one that hated fighting and training, surviving conflict with Frieza was considered impossible. Yet, for Trable to have survived both Abo and Kado’s attack, he must have been roughly, if not a little stronger than what his brother Vegeta was at the end of the Frieza saga.
Therefore, Tarble is much stronger than a Saibamen and that makes him stronger than Raditz. However, Raditz would be victorious if it came down to battle tactics and skill. Though, Tarble’s inner strength would likely overwhelm Raditz if the two were to ever fight right now, assuming Raditz hasn’t grown in strength while dead that is.