Erik, contactors normally have two sets of contacts with the bar operated by the electromagnet whereas a relay has only one set of contacts for the circuit. The contacts are less likely to weld in a contactor because of this.

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O | O appologies for the simple diagram.
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U Kaz, lets take something like a washing machine. The older ones used triacs and/or relays to control the motor.The smarter ones used a relay to control the main motor and triacs to control the solenoids. Why? Solenoids have nowhere near the dynamics of a motor - if the seize they don't try to pull infinite current and they don't get overloaded in operation like the main motor would. That makes it easy to engineer the triac and the attendant circuitry. The inductance is known for the solenoid so the amount of energy 'kicked back' can be quantified and a suitable snubber circuit designed. Also the currents involved for the solenoid are relatively small. In this instance the triac circuit will be very reliable (assuming it is well designed). As for the motor circuit - in a washing machine we know what motor is being used but it is most likely to be overloaded at some stage in its life. Also, the current drawn by the motor will be around 5A. The startup current of a motor is much higher. Along with the dynamics of the motor this makes designing a commercially viable and reliable triac circuit much more difficult. The fact that people like Panasonic make a relay specifically for this application tends to suggest that there is a market need for such a device. The JN series relay is economical and robust as hell - I have not seen one fail. Even when dumbass customers changed the circuit breaker from 10A to 25A on a 10A dimmer circuit - the inductor failed (it was good to 16A) the relay (JN series) and the thyristor pair (no triac in this instance) survived. The customer complained the dimmer was 'unreliable'!!! Sheesh. They 'thought' that changing the circuit breaker rating would give them a higher current dimmer!

So, if you REALLY want to use a triac - go ahead. If the customer is going to wire in the motor (that you have no control over) odds on they will short the output or do something that will vapourise your triac. They will complain bitterly about your faulty equipment (even though they zapped it). The signs of an abused relay are easy to see so at least you have some evidence to call your customer a numpty. Using Triacs isn't rocket science but the devil is in the details. No insult intended, but when you asked whether to switch the active or neutral tends to suggest your skill in electronic design is probably at the level of selecting a suitable relay.

At no point have you told us anything about the motor. I'm assuming something that draws a few amps - if its a little fan motor - then use a triac!