Something else to take into consideration is whether you will be selling your services as a renderer to clients or if you are just looking to jazz up your own work... and also whether you intend to incorporate other 3D software into your workflow or use Sketchup exclusively.
If your are the primary consumer and Sketchup is your only 3D app then I would base my decision not on quality or price but on how fluid it is to use within Sketchup... life is complicated enough without making it more-so unnecessarily.
If you think you may end up selling your render services at some point then probably best to go with one of the more full-featured pro-level apps (V-ray, Maxwell, Indigo, etc.)
Also worth considering, an app that is stand-alone can be very useful for very large/complex models/scenes that Sketchup cannot easily handle.
Best,
Jason.