At the moment the factors that affect your decision include
A) Budget > Open source, several hundred dollar software, or top of the line renderers that are $1000 or more.
B) Available Hardware > CPUs available, GPUs (Cuda/OpenCL), RenderFarm, Online Renderfarm Service
C) Raytracing vs Scanline Render and Reyes Rendering (Renderman) > Cars benefit from Raytracing, but scanline rendering uses cheats to look realistic, rather than doing the brute force calculations of raytracing. Raytracing allows for more realistic reflections and shadows, as well as glass and caustics.
d) amount of time to learn the software > programs like LuxRender are easy to learn, because you are simulating reality, while programs like renderman require a lot more technical knowledge. On a scale BI is closer to renderman, and takes more work to look good, while Cycles is closer to Luxrender, and takes less work to get photorealistic renders.
e) Are you going to use layers to composite the final scene at the end, or will you render as is. Luxrender does not allow layers, although it does allow light groups. Cycles does not have layers yet, but it is on it’s way.
A review of Open Source software that I am familiar with.
Cycles is incomplete at the moment, so there are a lot of features missing, but It can render cars and metal ok already. Layers are coming soon so you will be able to render images as separate passes and composite them. It is Raytracing, Montecarlo method, and attempts to be along the lines of Arnold Renderer. It is CUDA enabled or CPU only at the moment. It only does path tracing, so it takes a long time to render scenes lit with bounce light. If you have the personell, time, and resources for renderman, it is a great choice, but from your first post, it seems like Renderman might be a stretch for you in this next production.
LuxRender attempts to be completely unbiased, so You cannot render layers and you can’t do any tricks with it. It uses brute force raytracing, and has a wide selection of realistic materials to choose from. It has OpenCL GPU acceleration, or CPU. You can use light groups, which allow you to relight the scene after the render, but you can’t split diffuse, ambient or specular passes.
BI is a scanline renderer. It is old, but relatively full featured. It’s not realistic by default, so it takes a lot of work to get good results (relative to cycles). By default BI is faster, but as the quality improves it ends up taking longer.If you are really doing Car animations, this is probably not the choice for you, but BI makes an excellent tool to supplement many of cycles shortcomings by creating additional layers.
Yafaray might be a good choice. I have not used it, but It leans closer to Cycles and Luxrender. Some say it is faster, and therefore might be better for animation than luxrender, but it is not as realistic out of the box.
Renderman is a Specification created by pixar not a software. Pixar sells Prman as a renderer that does the Renderman spec. It is $2000.00 (price may be wrong). But since it is a spec, there is also 3Delight, where the first two cores are free. But the open source versions are Aqsis and Pixie. Prman is used in most of the major blockbusters that come out today, and the spec allows you to get similar results from any Renderman program. There are two catches though. First Renderman is complicated and takes a lot of training to learn (Some say years). Second, Although Prman has a new Raytracer that is fast, your mileage will vary with the affordable renderers when it comes to raytracing.
I hope this helps.