as for constraining shutter speed versus aperature, depends on what you want out of your picture. just remember the faster film will be more susesptible to bleeding from longer exposures. I'd treat each film separately and go for what would give you a proper exposure. then compare the negs and the prints afterwards. Regardless of keeping the dof fixed, you are going to see noticeable differences in the final results from the film speeds. Most film manufacturers will claim that the technology has advanced so that 400 speed is very sharp, and it has, but they continue to make the slower speeds for a reason. especially in the professional series. Most macro shots look best when the image is sharp, especially on enlargement. Having spent 22 years in the field and doing a lot of experiments, I'd always opt for the slowest film i can get my hands on.