The 6 will be twice as big of a dot, I personally like a small dot, for more precise shots, I can always turn it up to acquire it quickly, or during high stress
The MOA ( minute of Angle ) is an angular size measurement. That is approximately equal to one (1) inch at 100 yards. So, the 3 MOA dot will cover a 3" circle at 100 yards and the 6 MOA dot will cover 6" at that same distance. Be careful here. This same 6 MOA dot will only cover 3" at 50 yards and 1.5" at 25 yards and 3/4" at 10 yards. This pistol is a close range tool, mainly for use across the room or so. Inside of room range the 3" MOA dot will be about 1/4" in size and the 6 MOA dot about 1/2" in size. The idea is to find the sight fast and in short yardage range, bigger is more than likely better. Faster and easier to pick up when you need it the most.
If we were talking about putting a red dot on a full size precision gun I would steer toward a 3 moa dot. But, and this is a big but, since it’s on a micro-ish size gun I use 6 moa. Here is why. Self defense is usually within short range, but let’s exaggerate and say 50’. We both know this is to far to shoot a little gun and hightailing it out is most likely the best choice. But I digress. At 50’ that dot will equal 1” on your target. We are not splitting playing cards here. Your dot will be twice as easy to find and see over the 3 moa dot. That’s real world input for this particular choice.
I also prefer the smaller dot. You can always turn up the intensity and make the smaller dot appear larger but You cannot make a larger dot appear smaller
It fits, but it doesn't mount. you need shorter screws. They need to be 7.5mm long. Look it up on reddit. It can be done.