It reads like the start of a US Sitcom joke -Arnold, Ray and Max.
We are referring though to 3DS Max, the previous default rendering engine -Mental Ray and the new kid on the block – Arnold.
Arnold ships as standard with 3DS Max 2018 and Maya 2018. Mental Ray is conspicuously absent in the new versions of the software. Without delving too much into the “why” (possibly something to do with the fact that Nvidia owns MR and Autodesk owns Arnold) – Arnold is a full fledged rendering engine that has actually been used in numerous movies over the last few years.
Arnold offers a number of features that give it some advantages over Mental Ray. Firstly, in a lot of scenarios it is notably faster than Mental Ray. It is also very predictable in terms of lighting the scene (a bit like Vray in that respect). There are some subtle trade offs in render quality over MR but you can adjust the quality settings to get that extra bit of render clarity if you have time.
In real terms, having a render engine that can churn out renders faster and which will give you more certainty about the consistency of the output is a great thing.
Autodesk have included a tool called Sceneconverter that converts your MR materials, lighting, geometry, maps and cameras (and more) to Arnold equivalents. Testing has shown that this is quite robust.
There is a great deal of familiarity with the materials and render settings in Arnold. The render settings are a bit simpler and there are no GI or FG separate dialogues as such. There is a pre-made material library in a similar vein to the Autodesk MR Architectural Materials. There are some unique materials that are useful for recreating skin – this would be be one reason for Arnold’s expanding adaptation in the professional CGI industry.
One thing that you will need to be aware of if you plan on using Backburner (Render Farm) you need to purchase a node license for the render nodes otherwise you get a watermark in the rendered frames!
And if you get stuck or want some light reading then there is plenty of good documentation and videos that you can consult to get answers.
If you haven’t opened 3DS Max or Maya 2018 then it would be worthwhile installing them and playing around with some test scenes to see what you think. This is a really good real-world examination of the differences between the rendering engines – https://antoniobosi.com/maya-render-tests/render-comparsion-tests/arnold-vs-mental-ray-render
For an overview please have a look here – https://area.autodesk.com/blogs/the-3ds-max-blog/max2018rendering/
You can still get MR and Iray “plugin” form here – Arnold, Max and (Mental) Ray – BUT, it now has to be purchased.