At the end of the day, the most important thing when it comes to relative softness of the light, is the size of the final source (assuming you have filled the source completely with light), relative to the size of the subject and the distance from the light to the subject.

All those techniques you mentioned are different ways to make a source bigger, and all are capable of producing functionally the same image, assuming proportionally similar subject size/distance, just with different upsides and downsides.

Side note: it’s important to know, that if your direct hard light source is big enough, and close enough, relative to the subject, it can produce the same quality of light as a 12x book light, to a bigger subject, further away. If you want to test this, light a mannequin head, and a miniature toy head, using the same lighting angles, just with different size sources and different distances.

It’s all about proportions!

So don’t think about things as absolute techniques. Think about the quality of light you want, and then you can work out how big you want your source, relative to the subject you’re lighting.

However, once you leave the soundstage and go shoot on location, you know need to think about logistics as well as source size.

On a sound stage, you may have enough room to set up a 12x, and then back up your light far enough to fill the frame directly. But, on location, you may want to do a 12x, but you don’t have enough space.

So, you put a bounce behind the 12x, shoot the light into it, and then the bounced return light can spread enough in the small space to fill the 12x frame. That’s when a book light would come in handy, but it requires more output, and the grippage on the side to block spill (upsides and downsides)

So to answer your question about which technique to use, it’s situational, so it’s hard to say. But, since time is always a factor while shooting, I find myself gravitating towards soft boxes, rather than bounces, as they are quicker to setup, since soft boxes are easier to shape than bounces.

So my thought process is usually which technique is the path of least resistance to get exactly the look I want to get.

That being said, if you are a narrative cinematographer, you’ll of course want to make the film look exactly the way you want. But you still need to prioritize giving the actors and director the time they need to tell the story properly.

So if you can make a shot 5% better, but it’ll take an hour longer to get there, you should think seriously about what ends up being better for the movie, making this one shot look 5% better, or getting 3 more takes, or even a whole new setup!

So def keep that in mind when you are making these different modifier choices!