I have both. The 6 MOA dot on the 407K is nice for fast shooting. The Circle dot on the 507K is nice for a more precise shot with the 2 MOA dot. On a gun small enough for the RMSC footprint, I would say that the 407K for $244 on Amazon is a better option that the 507K for $296 if you are watching your penny's.
As far as green vs. red, it all depends on what your eye picks up better. I can see the red dot easier on a green background than the green dot. Plus, my night sights have a green dot, so going with red makes more sense to me to save confusion.
As far as green vs. red, it all depends on what your eye picks up better. I can see the red dot easier on a green background than the green dot. Plus, my night sights have a green dot, so going with red makes more sense to me to save confusion.