I would say this is purely dependent on the budget. Just buy the graphics card with the highest benchmark scores you can afford. Or you can also consider other properties, such as power consumption.
It probably makes sense to get an NVIDIA card, because you will then be able to use the Optix denoiser.
It probably does not make sense to use multiple GPUs in SLI/NVLINK/other configurations, as the Optix denoiser sometimes throws errors in such systems. This should be fixed in the newer version, though (which will be implemented in Corona ASAP).