Difference B/W using a Contactor and Circuit Breaker for A H.T Motor?

Opening and closing = movement = wear and tear.

Circuit breakers are meant to open occasionally in emergencies, contactors are designed to be opened and closed frequently.

Circuit breakers are more expensive.

More wear and tear, not designed for the use, higher cost = higher cost of ownership, making it a bad idea to use a circuit breaker unless there is no other choice, i.e. they don't make a contactor large enough for your application.

Also, there is a phenomenon called "current chop" in vacuum breakers that adds to the "bad idea" column. When a vacuum contact is opened, the current flow can stop precociously, causing a ringing effect or "chop". This causes a high frequency pulse to occur and can set up capacitive coupling in the cables that becomes a high voltage spike at the motor windings. The contact material in vacuum contactors is slightly different than that used in vacuum circuit breakers in order to mitigate this. Using circuit breakers then can lead to motor winding insulation damage due to current chop more quickly than if a contactor was used.