There are a few similar threads to this.
The only notable differences, if the rumours are to be believed, are the smaller notch, marginally improved battery and processor performance (I've read approx 8-10% improvement in battery capacity, not necessarily translated to 8-10% longer battery life), and cosmetically it'll be fractionally thicker with diagonal camera lens layout on the back.
Honestly, if you're buying new and want to stick with a mini-sized iPhone for the forseeable future, then you may as well wait until the 13 Mini is announced. If it's the same pricing as now, I'd go for that if you don't have a 12 Mini currently. Might as well buy brand new, the latest and greatest etc.
If someone currently has a 12 Mini (like myself), it's getting harder and harder to make a case for upgrading. I was absolutely set on upgrading but with such lacklustre/underwhelming changes, it feels almost stupid to sell my device and have to pay a few hundred £s/$s for essentially the same phone with trivial updates.
120hz on the Mini would have convinced me. TouchID under glass would have convinced me. Removal of the notch completely (or pinhole) would have convinced me. A considerable form factor change would have convinced me. But none of those things are happening this year apparently so there just isn't an incentive.