Should a person with prediabetes consume soluble or insoluble fiber?

Both: Soluable fiber helps with lowering cholesterol, insoluble helps with passing normal stools. These are both laudable goals... So eat it all. Oatmeal and almonds have both, so enjoy.

From food is best: It is always better to get fiber from foods (fruits, vegetables and grains), than from a supplement. For one thing, it fills you up and so you eat less protein and fat (high calories, less fiber usually). High protein diets are often low in fiber, and studies showed in the past and recently that they are not as healthy as many think. Besides fiber, there are other essential nutrients with them.