Here is a list of all the features of both products.
Here are some more comments from Rich:
We renamed the plugin nXtRender to ArielVision to help avoid the confusion between it and IRender nXt.
Basically, ArielVision is a plugin that uses the same rendering engine as IRender nXt, but has viewer features.
The difference between ArielVision and IRender nXt can be seen on this comparison chart:
The products are described on these pages:
You can download and install both plugins, and run them both at the same time in SketchUp, to see the different features on the toolbars and dialogs for each product.
Thank you for your interest,