[FRpilot]

by the way you formated your questions, i dont even think know why one caliper would better than another. 30mm is the space between the caliper that allows clearance for the rotor. generally, it doesnt matter much, but a slightly larger rotor widthwise will offer slightly better heat dissapation and cooling.

you would want to look at how big the caliper is and how many pistons it has. if the pistons are bigger and the caliper takes up more space on the rotor, then it should allow more of the rotor to be 'clamped' down when you apply the brakes which would help your car stop faster..

as to which calipers would be better, it depends on what you are using the car for. it is up to you. if you are just trying to be cool, then get z32 calipers since thats what ppl usually get . 180sx brake rotors are only slightly bigger than stock 240sx rotors. most ppl find that q45 brakes are adequate for what they use the car for. if you want to go bigger, go with z32 calipers, or if you want to go all out, go get some 3000+ big brake kit with 6pistons, 13inch rotors, etc by Endless, project mu, etc.