Here is some info from a BMD camera specialist comparing the two camera models.....

Blackmagic URSA Mini Pro 4.6K G2Blackmagic URSA Mini Pro 12K

Please see below....

"First thing that I would like to straighten up, is that neither the Ursa Mini Pro G2, nor the Ursa 12K have dual native ISO. Both of these cameras have one native ISO. Currently only our Pocket and Studio Cameras feature the dual native ISO

When it comes to Blackmagic Raw, although you are right, that the Ursa 12K doesn't support ProRes recording, the workflow of Blackmagic Raw is very similar to ProRes. You still get exactly the same dynamic range (gamma + colour) options and it does not require any more work than ProRes, although, when necessary, it allows for extra flexibility when certain shots have been incorrectly exposed then Braw has that extra level of safety as it allows to change ISO values in post production, or even completely change the colour space should the user want to dive deeper into colour grading options. It is important to understand that this however is not required - one can simply leave everything as it is, and add the same LUT that works with ProRes

The benefit of Blackmagic Raw is that it's 12bit in comparison to ProRes 422 codecs being 10bit

When it comes to differences

Ursa G2 has a slightly higher dynamic range: 15stops comparing to 14 stops

Where Ursa 12K shines is the colour reproduction, especially in skin tones, thanks to Gen5 colour science and new, full RGBW sensor, which means that every pixel has the same amount of Red, Green and Blue subpixels, and extra added White subpixel for luminosity. The Ursa G2 has a more common, RGGB pattern sensor (twice as many green subpixels than red and blue ones)

Ursa 12K however does require significantly more storage when shooting in full resolution so this may be a very important factor, especially for documentary style shooting

When it comes to low light capabilities - the two cameras perform similarly, Ursa G2 may have a slight edge here due to lower pixel density, but neither of these cameras are designed for low light operation. That doesn't mean that they are not good in limited lighting conditions. They are not however designed to be used in very dark situations, like dark night with no extra lighting, there are better tools for this kind of job."