I would just point out that it's about 160 miles to Sarzana, so what Google Maps says about 2 hours and 36 minutes to 2 hours and 56 minutes with no traffic, which may be right once you are in the car, and there is no traffic. It will take some time for you to get in the car.

Most of the car rental agencies are in Terminal 1, where I assume you will be arriving, and there may well be a line, then you will have to deal with the clerk for 10-15 min, then walk 10 minutes to the garage where the cars are kept. So you might want to add a half hour or so to get into the car

However, if you book an agency that is only in Terminal 2, or one that is "located" at Malpensa but does not physically have the cars in terminal 1, you may want to add half hour or more. I've waited up to 45 minutes for hotel shuttles at MXP on several occasions, including a week and a half ago.

Depending on your arrival time, you may hit traffic. In Milan the traffic is bumper to bumper terrible. To get from MXP to Sarzana you basically have to head straight towards Milan, so if there is any traffic, you will be caught in the middle of it, both heading towards and driving away from Milan.

In your economic considerations in addition to gas and rental fees, remember that you will also most likely be on a toll road.

Finally, in totaling up the time it will take by rental car, consider the time it will take to return the car. First, you have to find the rental agency in Sarzana. That may, or may not be close to the center of town. It is possible that you will have to take a cab from the car rental agency to your hotel. Add 30 minutes for the time you will spend waiting for the cab to show up. You'd probably have to add time for a cab if arriving by train, but unlike rental car places, the train station is always downtown, and there are always cabs waiting at the train station.

Finally, the last time I rented a car in Italy was this past July. It took close to an hour to return it once I got to the travel agency. Why? Because they were closed. They were on "pausa," the Italian equivalent of a siesta. Most businesses close between 1-3:30, and Avis was closed, after I had driven 4.5 hours to get there.

You can do the equivalent of an after hours drop-off, and just park the car in the street, shove the keys through the mail slot, and walk away, but then again, you may not be comfortable doing that. It's what I did after I got tired of waiting and started banging on the door enough to determine if they were in there, which indeed they were, but they wouldn't interrupt the pausa and come out and accept the car.

Then, I did not have the phone number of a cab company in that town to take me to my hotel, and I since I couldn't ask the Avis people to call one for me I walked from store to store, asking people to call a cab for me, and of course, nobody knew the number. Finally I arrived at a hotel, and they called a cab for me, for which I waited about 30 minutes.

Overall, train versus car is a judgment call. Google Maps says it's 2 hours and 36 minutes without traffic, MXP to Sarzana, but these are some other things I'd consider in making your decision that Google Maps does not include.