Arcing will also happen when the contacts open,and on certain types of load can be more impresive than the switch on arc.As the contacts of either close they will arc
Another differences between the 2 is a contactor often has extra auxillary,lower rated contacts for control circuitry and as i mentioned above allows the addition of overload protection that link into the main and aux contacts without any extra wiring
Din rail mounting bases are readily available for the common octal and 11 pin relays,the advantage is you can just unplug the relay leaving the wiring in place ,were as you'd need to unwire the contactor.
One final thing to consider is the electric brewery is designed for american wiring,the need for contactors is due to the 240v being obtained from 2 phases,not phase to neutral,only switching 1 phase wont totally isolate the mains.