The 12mm has a 12mm lens and lower base mag of 1x (up to 6x), so you will have a wider field-of-view on your display. The 16mm version has a magnification range of 4-24x and a smaller field-of-view, this makes it suited to long-range night-time shooting.
Commander NV100 12mm Version
The 12mm version has a base magnification of 1x and magnification all the way up to 6x.
This makes it ideal for close-range shooting of anything less than 50 yards, so it's more suited towards airgun and rimfire setups.
The lower base mag also means you'll have a wider field of view on the monitor, so shooters can quickly track and identify close targets.
Commander NV100 16mm Version
The 16mm version has a magnification range of 4-24x, making it ideal for centrefire and long-range night shots.
The only downside to this is that you have a reduced field-of-view.
However, you will most likely be shooting at targets at least 50 yards away, making this field of view sufficient.
Image Comparison
The distance between NV100 and the target is 4 meters, the image 12mm (Left) vs 16mm (Right) as below:
How to distinguish 12 and 16mm nv100 from appearance
The only difference in appearance between 12mm and 16mm is the barrel focus ring, the 12mm thread is shorter, see the picture below:
Commander NV100 12mm: https://oneleaf.ai/commander-nv100-night-vision-add-on-12mm.html
I've also drawn up a design for a single scope ring that's adjustable from 50mm to 40mm with a lock ring. Until then using small diameter/wide rubberbands work very well for filling the gaps and no marring. Love it, mean it, and look forward to future purchases!