Taiji is a martial art. Like all martial arts, it is about the manifestation and application of power. And like all martial arts, power comes from the speed generated when one is relaxed.
Force = mass x acceleration
At any point in time, our mass is fixed, so that only thing that we can change is acceleration. This is the point of being relaxed. Being relaxed helps us to go from zero to a high speed in less time. This gives a higher value of acceleration compared to being rigid.
The goal is the same: to be relaxed so that you can achieve the maximum acceleration.
The way to get there is usually split into two: external martial arts (外家) and
internal martial arts (内家). In simple terms, external martial arts focus first on speed, then work toward being relaxed while maintaining speed. Internal martial arts focus first on learning to relax, then moving toward using that relaxed mode to achieve speed.
Which means that, at the end of the day, there is no "better" method. Instead, it is about understanding the training system and concept behind the martial art you practise, then keep practising it.
Side note:
While the goal is the same, the journey is not. The application of martial arts always requires speed because no one is going to attack you in slow motion. So if you are able to achieve speed, even if you are not relaxed, you will still be able to somehow apply the martial art you practise, even though not to maximum effect. But no matter how relaxed you can be, if you cannot turn that into speed, you are going to get beaten up when attacked.
