I used both in 3dsmax, you can hardly compare these two products without understanding the fields where they operate. Corona render is a standard of architectural visualization!
Fastest Cpu render plugin and reliable on huge scenes, can easily setup as mentioned before (one click render), tons of premade materials and tutorials for Archviz.
One real competitor is - Fstorm (which is also oriented in Archviz-3Dsmax).
Redshift in another hand is a best choice for vfx, motion graphics, films. Fastest and reliable Gpu render plugin with great functionality, you can tweak almost everything what you need in almost every software.
But doesn't have enough premade materials, tutorials, pages or communities for 3dsmax.. "rocket science setup" in the eyes of Corona user.
Render legion should work in the direction of Gpu render, otherwise they will start to loose step by step to someone like Lumion.. One click render feature heh? How about one click project? :)
Instead of developing plugin for Cinema4d, you guys should go deeper into Sketchup.. adding more functionality for vfx only will harm your product. And let's be honest, its really hard to beat the titan like Redshift(GPU) in Cinema4d or Houdini.
Wish you will stay in Archviz field :)