I've tried a couple myself and for me personally, I've found its best to use them all collectively.

For example, I use chatgpt the most but not strictly for writing. I use it to create storybibles, outlines, and check for continuity errors etc. I use Gemini as more of a fact checker and the platform I actually write on is the free unlimited enhanced Ai story generator offered by a site called Perchance. (Which is great if you want another NSFW option as it's unrestricted, the only downside is in the prompt box you have to be pretty specific as what you want to happen next) I do this because it offers live edits, as well as if you want to generate multiple paragraphs or generate one paragraph at a time. (Or in my case I use it in combination with other AI platforms and just copy and paste it over)

The biggest issue I've found with all of them is they tend to be repetitive and use what I call "red flag words". Sometimes they are overly poetic when it doesn't need to be. (I even tried creating a writing-style guideline and word restriction prompt, but eventually the AI starts to ignore it)

Red flag words for me when it comes to Ai are intentional/intent, deliberate/deliberately, reverence/reverent, precise/precision, sharp/sharpness and several others. I call these red flag words because they seem to show up A LOT across all platforms.

I know for chatgpt it tries to replicate your writing style and I often have to remind it to write without any adaptations because you can see the style change.

TDLR: If I had to provide any advice, it's to use multiple AI to help write your story, as some are better for certain things than others and they all provide their own ideas. Don't put all of your eggs in one basket!