I have both. For me personally I use iPads for a lot of things. I have two currently (a 13 inch pro and a Mini 7 but have had pretty much every other size. My personal take is:
13 inch is great for people using it as a secondary (or even primary) computing device. I used mine heavily (before they changed the rules) for all my work stuff that involved Office/Teams. It was fantastic for that and kept those distractions off of my main work computer which is for coding. I also use the 13 inch as a secondary monitor through sidecar, especially useful if I'm just using my laptop. I also use it for photo editing apps, art apps, etc. It's great to use with the pencil.
iPad mini is great for people who want a fantastic travel companion, or a smaller device for reading, watching videos, gaming (which it excels at size wise), or just casual surfing. I use my mini all the time for this and just love it for travel where the 13 inch gets a bit unwieldy. It's also great for art apps you don't need a lot of space for (like pixel art apps). Also although I have normal sized hands, people with tiny hands absolutely love this size as well.
Although you didn't ask about it, I think the 11 inch is the best overall size for most people if they're unsure. It's not terribly big, but it's bigger than the mini if people find that too restrictive. If someone wants to use an iPad as more of a computing device vs an entertainment device but still want it portable for travel (and have an optional keyboard) I think the 11 inch is a great iPad size.