"Why is one's render better than the other?" - Because that person made a better render. Likely because they understand materials and rendering better, or just cared more about the render.
As to which is better, meh. It's difficult to compare because they are presented differently, and show different details (one shows wireframes, the other doesn't). But neither are particularly good. They're both vaguely blobby and don't appear to be well constructed, with shading errors and an overall lack of detail or precision.
Here's a random one on the same site that looks much nicer:
You can see how well constructed this one is, and how accurate and crisp all the edges and panels are. Check out the wireframe shots - the topology is clean and well thought out. Presentation is still not amazing here, but it does the job and shows the model well.
@EarthQuake really gives a good answer, the topology on the first one is really gross and the second doesn't generally strike me as any better. The shading issues are a little gnarly on both...
For 'class A' surfaced objects, a high end render pass from experience is typically dependant upon an optimised topology design, although worth remembering minor shading errors can be fixed in post:
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As to which is better, meh. It's difficult to compare because they are presented differently, and show different details (one shows wireframes, the other doesn't). But neither are particularly good. They're both vaguely blobby and don't appear to be well constructed, with shading errors and an overall lack of detail or precision.
Here's a random one on the same site that looks much nicer:
https://www.turbosquid.com/3d-models/tesla-s-2014-rigged-car-3d-model/778066
You can see how well constructed this one is, and how accurate and crisp all the edges and panels are. Check out the wireframe shots - the topology is clean and well thought out. Presentation is still not amazing here, but it does the job and shows the model well.
https://www.carbodydesign.com/article/59531-modeling-cars-in-polygons/