As someone who has used and tested both the 12 and 13 mini quite a bit, here are the only things that are NOTICEABLY different between the 13 mini and 12 mini:
- The battery life on the 13 mini is noticeably better/longer-lasting than the 12 mini. I'd say I'm getting on average an extra 1.5-2 hours a day more.
- The 13 mini is noticeably a little heavier than the 12 mini
- The diagonal lens arrangement on the back is different to 12's vertical arrangement
- You get those nice exclusive live wallpapers on the 13 mini (and 13) that you don't get on the Pro or any other iPhone model (including the 12 mini)
In terms of what is noticeable between the 12 and 13 mini - that's it. In real day-to-day usage it isn't any faster or any brighter, the camera takes similar quality pictures and the screens look great on both. Those bullet points are the only things I can genuinely say are noticeably different between the two. Even the notch size difference isn't really noticeable. I used the 12 mini again just now and didn't notice the increase in notch size at all.
In terms of whether a 12 mini is worth it - it comes down to one consideration. Battery life.
If you're a 'power user' dependent on tons of battery life, perhaps coming from a larger phone, that is constantly streaming stuff, using lots of widgets and apps, bluetooth and other things that run the battery down, the 12 mini is NOT for you. If you rely on your phone that much then you'll probably be charging it at least a couple of times a day.
If you're a casual phone user, occasionally browsing the internet, checking/sending the occasional message, not streaming video all the time then the 12 mini is a great phone and a bargain at the moment used. In the UK you can pick up a mint condition one for around £400 or less at the moment.
The 13 mini isn't exactly for power users either, but it comfortably has a full day's battery life for the average user. I'm very impressed with the 13 mini.