I am using an X and I differ little bit over here on keeping the X vs buying a new phone.
I am surely going to buy 12 but, will opt for higher storage mini version of it. It has everything that 12 has, the size is compact and also, the screen resolution is better than X.
I also feel it will be little lighter than X as well (aluminum vs steel), the design is really nice. its very similar to the old SE model design and it feels really nice.
X is a special phone for me, will always keep it in functional condition and never sell it. But all technology items needs upgrading and for me, 12 is almost the perfect upgrade.
iOS is same everywhere but the things that helped me to make my decision for 12 mini are :
Better camera, better screen, even more perfect size for 1 hand operation, faster network connectivity, beautiful design and lastly fancy colors. I quite like the new color range.
What I don't like is battery capacity; But, with so many things to like, 1 dislike can be accepted (to me)
So for me, X goes to my display shelf and 12 mini becomes my daily driver when it comes out next month.