Excellent comparison of these AI systems. I personally don't like to use AI for writing unless it's to check facts, but I can see how these tools are immensely useful to refine and speed up writing, so I might integrate them more into my workflow.
The one AI app that I'm having fun with is called Reflectr AI Journal and Notebook (Android). This is hands down the most useful app for journaling I've ever come across. Basically, you journal throughout your day, and the AI analyses the content of your words and spits out advise and jokes that are surprisingly funny, useful, and accurate. You can also use it for to-do lists and note taking, but I use other non-AI systems for that.
They definitely speed up writing process but I am uncomfortable of using them. I only use AI for helping me with scheduling. It's so much fun. Have you tried Bing AI? it looks like SIRI but I am not sure which model it's based on. I'll check on the AI that you recommended, it sounds good!
I have tried Bing sporadically, and I liked how it adds citations to responses, which is useful for technical or scientific work. Reflectr is very funny and actually useful. Usually, I feel like journaling is speaking into a void, but this app gives feedback right away and then a summary at the end of the week.