When talking big power like this, less is better
30 Amps means that less current (hoh flowing through the pipe) is flowing than the 40 amps, or greater current.
That means that:
A) Motor Controllers Cooler
B) Fuses trip easier
C) You can use thinner wires
D) You fuse is smaller
E) You will ask yourself why a motor suddenly shuts down at ~75% power!