smaller second number means less top speed. this give you more pulling power at lower speeds(like hovering).

the first number is the length of the prop from tip to tip. the second number is how far it goes in one turn. so with the 12x4 the length is more but the bite is less. and the 12.5x3.75 is a little longer than that and even less bite which will give even less top speed and more low end pulling power. this is good for 3D. I think the 46 la should be able to spin this prop ok. the master airscrew is lighter i think. this might be better if you dont get the spool up you want but ild try the apc props first.