For a given image size, regardless of the format and focal length, there is a DOF for each f/stop.
A 1" image on in 35mm film will have the same depth of field for a 35mm, 50mm, 100mm lens. The perspective will be different.
A 1" image on in 4"x5" film will have the same depth of field for a 90mm, 150mm, 200mm lens. The perspective will be different.
You are photographing a sphere. Therefore shifts, tilts, ... will not change the DOF.
Now we have eliminated movements. The first question is what size image do you want on the film, and hence the amount of enlargement involved. This eliminates the question of grain as you specified in your first post.
The second question, given that either the 35mm or the 4"x5" cameras can be set up and focused on your subject, you can now take a longer exposure and increase the exposure time in a trade off for DOF. Therefore, which camera do you choose to use?
The smallest 35mm f/stop is typically f/16 with the best performance without diffraction is around f/8.
The smallest 4"x5" f/stop is typically f/32 with the best performance without diffraction is around f/22 or f/16.
In summary, for a given image size on film the focal length does not matter. DOF is a function of f/stop.
The object is a sphere and therefore movements will not help.
To increase DOF, lengthen the exposure.
So pick the camera which will work for you, given any diffraction issues that you have.