Hardboard and MDF are wood fibreboards, but they are not interchangeable for every part. Hardboard is useful for thin sheet parts. MDF offers a uniform structure for shapes, models and parts you plan to finish later.
Compare both surfaces
Look at both sides of the specific hardboard product. The front and back may have different textures, affecting which side should face outwards. MDF has a more uniform appearance, although coloured, white and brown versions differ.
Choose thickness for the construction
A backing panel or stencil needs little space. A holder, joint or freestanding part requires more attention to stiffness. Compare the available thicknesses of hardboard and MDF before specifying slots or edge depth.
A fully supported thin panel has a different purpose from a long strip carrying a load. Assess the sheet together with its supports.
Cut edge and finish
Both materials develop a dark laser-cut edge. If you plan to paint the parts, include their edges and internal openings in your test. The work after cutting can affect your choice. Read about preparing hardboard for paint.
Choose for your part
Hardboard may be sufficient for a thin model part; MDF may be more practical for a thicker slotted construction. Compare surfaces with a sample and test the joint in the selected sheet. Choose according to the part's purpose rather than the material name alone.