Hi there!
I use both. Below some of my thoughts:
- yes corona can save as exr, but Arnold's drivers system is way better
- AOVs/render passes - in corona you have to do it manually for every project/scene. No presets like in Arnold
- no cryptomatte in corona :(
- XP and Turbulence FD are not supported. Using xpGenerator is a nightmare
- no scatter system
- you can use corona instances but not as good as in Arnold
- material node system is great in Corona. Clear and friendly
- Corona supports almost all C4D native shaders
- Arnold has more special shaders
- generaly Arnold's materials node system is not as pleasant as Corona.
- lights in Corona have less scene option than Arnold, you can on/off visibility, reflections etc. but you can't set the strenght of those effects
- Arnold works better with big scenes
- Arnold is more stable, actually I don't remember when I have last crash w. Arnold. Corona likes crashing but I've nottice it only when Cycles4D is installed...strange but w/o Cycles Corona works well on my system
- renders from Corona look awesome, I like them more than images from Arnold
- Corona has great denoiser. Arnold's is complicated and gives worst results IMHO
- Corona is way faster than arnold (on CPU)
- lightmixer is incredible, I love it. A newest version of Arnold has imager for lightmixer but again, it's more complicated
- Corona has own frame buffer, better than C4D picture viewer
- you can save Corona render in it's own format and resume it later. This is great. Not sure if you can to that in Arnold
- Corona is ready to render out of the box. Actually the very first render looks great. In Arnold you have to dig throuh bunch of render settings
So...If you are in VFX and using special plugins as XP, TFD etc. forget about Corona. If not, you can use both for motiongraphics, animations etc... but only with a support of render farms - sorry Corona and Arnold aren't the speed beasts.
If you are doing stills - use Corona. Renders are mindblowing and you will get the results you want faster.
I use Arnold only for short intros/outros, not for stills. It's about 20% of my work. Rest is Corona. My default render engine is Corona, as well as the materials :)
Cheers!