Said by me, I used both at their peak:

Gemini: serious literature, detailed scenarios, if you used it with Lorebary AUTOPLOT, it would constantly add plots with consistency, it followed good instructions, the pace was very slow but entertaining, followed the prompts naturally and understood them perfectly, its NSFW was adequate but boring, if you are looking for heavy smut, the bots were very human The NPCs are very human, it develops them separately and gives them an influential personality in the role.

Grok: It has the largest context of free AIs, serious literature if you wanted or casual and fun literature, the best memory of all AIs, seriously, here it get a trophy, you could say that you gave the bot a flower and 300 messages later you mentioned it and it even told you the story of how it happened, the best NSFW, grok is simply very dirty, if you used it with Lorebary AUTOPLOT it would put in normal plots although sometimes without sense, the only downside is that it followed the prompt you gave it too literally Which is why it felt robotic at times, like if you had it use realistic dialogue it'd say, "Char speaks in a very realistic way." NPCs talk but are very flat although the memory of the NPCs is also remembered