I've used both 30 and 40 flap for CAT 3 landings I find that 30 flap gives a smoother landing as for seeing the lights at DH I can't say that 40 gives a better veiw than 30 in fact I don't think that there is much to choose between them.
So for me the runway length is the number one factor for flap selection as most of the places that I go GA performance is not a critical factor however if the runway might be slippery I would go for 40.