AI on Android is Gemini, and you can run it on iOS too. You can ask it all the same questions and compare the results yourself.
The main differences are Gemini will remember you and the conversations you had (the Siri app is just a history, but it does not learn from that history), offers Gemini Live conversation where you don't need to keep re-triggering the conversation with a hello phrase, and is generally more detailed (Siri will keep things more concise). Both do image editing and image generation, but I think Gemini's nano banana is still a ways ahead. (Apple is miles better than iOS 18 for images though. Just still behind.)
Right now, Siri is only accessing things in your Apple apps and Gemini can access Google apps as well as Instacart, OpenTable, Canva, and GitHub. Not a huge list of 3rd parties. (I am not on the expensive plan where I can test the agentic DoorDash stuff, etc.) Once Siri AI launches in September, there will probably be many more 3rd party apps supported on Apple's platform, which could give it a real advantage.
As far as personal intelligence goes: if you have all your data in Google's services, you will get better results there. If you have all your data in Apple's services, you will get better results there. I primarily use Gmail and Google Calendar, but sync them with Apple's first party apps, so my results are about the same from one platform to the other.
They reference the same sources and provide roughly the same answers, so Apple's claim that they used zero of Google's Assistant (which isn't what it is called anymore) is kind of marketing bull. Siri AI is definitely Gemini under-the-hood and it is kind of obvious when you play with both for awhile.
Apple was definitely behind and playing catch-up. They have mostly gotten there and have a real chance to pull ahead with 3rd party app support. However, this IS Apple. In two years, they will be behind again. Guaranteed. They are not an AI company and this is not their priority. Google will keep doing new things and will keep the best stuff for their own customers and not share everything with Apple. This might be the best it ever gets on iOS. Think of features like App Library that launched in iOS 14 and never got changed. Apple puts out (occasionally) good to great software, but then they move on to the next thing and often forget to continue iterating.
Already plenty of people doing a side-by-side. Here's a couple I found quickly. There will probably be many more come September.
How Google Gemini and Apple Siri responded to a standard set of questions - and what I thought of the responses
www.stuff.tv