[mkhorton]

There are a few differences, but broadly MatWeb is more useful for manufacturing and has a broader range of materials available (including plastics, extensive metallic alloys, etc.) and real world properties. These are materials you might purchase and use today.

In contrast, the Materials Project are computed predicted information on inorganic crystals (typically, ideal, on-stochiometric crystals), that might be used for many different device applications like solar, optoelectronics, batteries, etc. Many of these crystals will not be available to purchase and will need to be grown in a laboratory, and Materials Project is therefore much more focused towards active research into new materials.