A method like time of day is very constricting, and becomes more like a disability than an advantage. Consider werewolves that convert, like it or not, on the full moon. It has been used as a story advantage when the werewolf feels compelled to lock themselves up for the full moon, or usually does and cannot get there in time, etc. But the scope of the story is still very limited.
Emotional state is similarly constricted; Bruce Banner becomes the Hulk when he gets angry. We get tired of it; or at least I did. The episodes become formulaic, there are only so many trigger points, and in a way you know Bruce is never in any real danger, as soon as he gets hurt here comes the near-immortal Hulk to the rescue, and all it really costs Bruce is a wardrobe expense, embarrassment, and moving on to his next angry episode.
A method of choice is the most flexible option, and perhaps too flexible, because it becomes like a magic wand in the hands of an expert wizard: The solution is always there. If a physical object (device or potion or amulet or whatever) is necessary for the transformation, then depriving the character of the physical object can be a useful plot twist.
Another way of restricting a transformation of choice is to use a special reserve of some sort, magical or not. The transformation potion requires the golden nectar, if the supply is out, it's out. Or the transformation is physically brutal and exhausting, try it twice within 24 hours and bleeding lesions appear on the skin, try it three times and you may rupture organs and die. Then you can engineer the plot so this is exactly what has to happen. We see the hero in an emergency do a second transformation with the expected consequences, to prove they are real. Then later the hero has to risk a third transformation to save his mission, and we know he is risking his life for his values.
What you need to do is balance these opposing forces; you don't want his skill too easy to whip out, and you don't want it too hard to use when your plot line calls for it. In other words, you want the plot to flow naturally and not have to force it to fit specific times or conditions, and you also want the use of the skill to be relatively rare so the reader is looking forward to the next installment of its use. That won't happen if the skill is on display two or three times per chapter.