Would Medpor jaw implants give a more defined and sharp jawline compared to silicone ditto since Medpor is a harder material?

In most circumstances, facial implants are placed directly against the bone and have  soft tissue over them, which makes them un-palpable. The amount of definition and contour is really dependent on the size and shape of the implant. In my opinion, custom silicone implants provide the best fit and appearance. Good luck

It is a common myth that the firmness of a bone-based material has an effect on its external appearance. That is simply not true and makes no biologic or biomaterial sense. The durometer differences between silicone and Medpor, for example, are so slight that their ability to push the tissues has no difference. It is not like comparing a soft injectable filler and a former solid implant material. It is the shape and design of any solid implant that creates its external appearance.

The patient may not know the 'side by side' comparison of materials but there is a difference to experienced surgeons. You also failed to mention a third material - gortex, which is the more ideal of the three for the final appearance. As well even though you mention the "equal" size and contours"; it is not fully accurate. Medpor is larger stiffer and less able to be contoured and does not establish a "flat plane" between the cheekbone and the angle. Silastic is softer but has a slightly 'rounder' appearance at the angle. Gortex achieves the most defined look at the angle and is able to be easily modified and contoured at surgery. A flat plane model like look is a result. The tissue  ingrowth is what establishes the look in a superior manner.