The mechanical speed controller in the rustler has a servo, which rotates an arm, which changes how much power goes to the motor... and how much power is burnt off via a resistor.
An electronic speed controller turns the power to the motor on and off - at different rates, to achieve the desired speed. No servo, no resistor.
ESC is more efficient. You'll get more run time and better performance.
You dont need the ESC mount for the rustler. A bit of double sided tape onto the chassis works fine. Actually - i think its better than using the ESC mount, as the ESC will balance the weight of the receiver.
ESC easier to plug up than an MSC. ESC plugs into the receiver, connects to the motor and battery - and away you go!