Monza itself has a small handful of hotels, and they get booked out fast and priced accordingly for race weekend. Most people who've done this trip more than once end up basing in Milan instead, and the train math is the reason why: Trenord runs Milano Centrale to Monza in 9 minutes, with trains roughly every hour from 5:25am to 11:22pm. That's a shorter commute than plenty of people have to their own office.
Three areas make sense depending on what you want out of the trip. Right around Milano Centrale itself puts you two minutes from the fastest train to Monza and inside easy walking distance of the Hyatt Centric Milan Centrale, a 141-room hotel with a proper spa setup (Roman bath, sauna, Turkish bath, salt cave) if you want to recover between sessions. See live rating and reviews on Google Maps. The Hilton Milan sits a couple of blocks further out on Via Luigi Galvani, still inside that same walk-to-the-station radius, and tends to run a little more corporate and a little less design-forward than the Hyatt, which some people prefer for a race weekend where you're mostly just sleeping there. See live rating and reviews on Google Maps.
If you'd rather trade a few minutes of commute for a lower nightly rate, Cinisello Balsamo is worth knowing about — just north of Sesto San Giovanni, with the Bignami stop on Milan's M5 metro line giving direct access into the city center and a short connection toward Monza without needing a car. The BB Hotels Smarthotel Re Milano Nord sits in this area, a 43-room 4-star property with a garden and terrace that's meaningfully cheaper than anything near Centrale, at the cost of a couple of metro/train changes to get anywhere central. See live rating and reviews on Google Maps.
The bigger case for Milan over Monza itself is what happens outside race hours. Monza empties out at night during a normal week. Milan doesn't. You get actual restaurants, actual nightlife, and a city that isn't purely organized around a racetrack, which matters over a three or four night stay more than people expect going in.
