Which is more important – speed, power or the ability to change?
Actually, this is like asking which of the three hand shapes are more important in the game of paper, scissors, rock.
Actually, all are important against the correctly guessed hand shape that the opponent assumes.
Thus, a good kicker can nail you before you can get close but a good wrestler can close in quick and take the kicker to the ground. However, a good boxer can prevent a wrestler from coming near enough to use his takedown techniques.
Similarly, you can blitz your opponent with your fast techniques. But if you fail to finish the job quickly you may run out if stamina. Or you can go the distance but still lose because your body cannot handle the hardened limbs of the opponent.
You can also throw 5 fast punches at the opponent but if not backed by power then that one retaliatory punch from the opponent backed by strong penetrating power can stop you in your track.
This is why in training Tai Chi we should not focus on one thing at the expense if another.