I love my 12 Mini, wouldn't trade it as my main device for anything.
Now I bought the 13 Mini on sale, not as much savings as I would have hoped with between 90-130€, but because of chip shortage etc it is even a wonder to get it on sale at all. Now I got two 13 Mini because I bought one a long time ago on sale not in stock and it shipped the day my newly ordered iPhone 13 Mini shipped too. :-(
The one is a pink 13 Mini 512GB, the other a 256GB white 13 Mini. And I chose the pink one even thought I don't need that much storage. Because of display quality. It is still crazy how different it is inside a line up. My 13 Mini 512GB has a way better display quality. Especially when comparing it to my other 12 Mini and 13 Mini when shifting the device you see the difference. It has more contrast but most of all is way cooler, a little reddish tint but just a little. The other 256GB 13 Mini is WAY more yellow and has this infamous yellow tint. You see it in normal handling but more than that when shifting it to the right or left. It is a night and day difference. My old 12 Mini has the worst display compared as it seems of low quality in general when compared. It has less contrast, yellowish tint like the 256GB 13 Mini and in general you can see that it has a worth quality than both 13 Mini displays. And I tried 5 12 Minis back then in several months before getting this device which had betterd isplay than the others.
So display quality difference is still there. I really was shocked how different those displays are. Thought they would have worked on it. So happy with the 512GB display. Is it a coincidence that the most expensive model with 512GB has the best display? It costs me 260€ more than the 256GB device but the display is what I look at all day so I chose the 512GB, also because I wanted a new color...
In general not much difference to my 12 Mini. A little more stable 5G connection, also faster. I feel like reception/signal is better in general as I travel a lot through low signal areas too. You feel it has a better modem in it, very important to me as a traveller and as it is the last Mini device for longer time. I get more constant connection. I also do feel like call quality seems a bit better and clearer. Besides that and the better display quality and the better battery and a bit faster UI, I haven't seen any difference yet. Haven't used the camera really in the last days I received it. But I still would recommend everyone to buy the 13 Mini instead of the 12 Mini as it is the last Mini and everything seems a little faster and better, esp modem and battery. But PLEASE check display quality before keeping a phone.