We generally recommend using MeshLab for viewing the full resolution OBJ files. Download the OBJ, MTL and JPG texture into the same location. Then download and open MeshLab. Use "Import Mesh" to select the OBJ and the other files will automatically be detected. This will allow you inspect the differences between the full 3D and the 2.5D versions.
The model (OBJ file) is separate from the texture (JPG file) which is what provides the color. The material file (MTL file) describes how the two are related.
When uploading to SketchFab you have to include the OBJ, MTL and JPG texture files. SketchFab limits the amount of model faces that can be uploaded depending on what plan you have. You can used MeshLab to decimate the model to an appropriate number of faces before upload.
The 3D model preview came out worse on your second job because it was processed with the full 3D option. The Full 3D option has much more detail associated with it but is still limited to being 10,000 faces in the preview. That is why it looks worse. The model will be better when viewed in a proper viewer like MeshLab.