Thank you all for your replies!
-13901: Glad to hear you have had fine experiences with the bus. It's always hard to gauge reviews.

-BobFF68 and Concerto: It's good to know that there are so many train options. I always like to have a plan B, especially on the way to the airport.

-bhomburg and JMN57: Thanks for the heads up on security/passport control at MXP. I have a USA passport (so not eligible for Fast Track), am flying into the UK (non-Schengen), and might have to check a bag, so it sounds like I should get there as early as possible.

-GUWonder: Autostradale/Arriva timetable shows departures every hour from Turin (https://www.airportbusexpress.it/Ima...AAeroporti.pdf), but I got concerned when I read reviews about buses leaving very late and turning away passengers who had bought advanced tickets.

Thanks again, and happy travels to all!