ISO 125 or ISO 400 film: which should I use for studio shots?
By the way, I am planning to shoot in black and white.
That would really depend on what kind of effect you are going for, and which format you shoot. Some can't stand the more apparent grain that comes with higher speed film, but I think it adds character. Then there is development and printing, etc. I personally find B&W ISO 400 film to suit my taste though, mostly T-Max or Tri-X with a side of Ilford HP5 Plus, and I use lower ISO film like Ektar and Sensia for colour.
Either way, when it comes to film, it is largely a matter of taste. I have seen Tri-X pushed to 1600 for studio portraits to good effect, and some have done great work with Ilford FP4 (ISO 125), or even Agfapan 25.
If you have difficulty deciding, I suggest trying a roll each, then see which you prefer. Film is pretty cheap these days :)
Depending on your lighting setup, you might find ISO 400 too fast.
I've shot in a smallish studio with powerful strobes, and even with the strobes at their lowest setting and shooting at ISO 200 I had to stop down to f/22.
This obviously isn't a problem if you're using flashguns, or if you have a big studio where you can position your strobes further away. Another option is to use an ND-grad filter in this case.
On one hand it is certainly a matter of taste, but the reason there are so many ISOs of film available is because they each have their uses and trade-offs. The higher the speed film, the more grain will be seen on an image, and this is especially noticeable as you enlarge the image or crop significantly.
Typically, studio work is a much more controlled environment, with lighting, camera stabilization, precise and known distances and framing. In this case, since lighting can be easily added to provide any desired aperture or shutter speed regardless of ISO, lower speed film can be utilized without sacrificing creativity. In addition, one must look at the application and destination of the image. Slower speed film has less grain, and can be enlarged (and cropped) with much more freedom before there are negative effects. Therefore for print and pre-press work, lower ISO provides much more latitude without resulting grain.
Of course, if you prefer the grain, ISO 400 is then a design choice, not a trade-off.