8 year old son wants to go to Ferrari & Lamborghini museums. Which is better?

Assuming you mean the Museo Casa Enzo Ferrari in Modena (not the Museo Ferrari Maranello) they are nearly identical in terms of access.

Either way you would take a fast Frecce train from Firenze SMN to Bologna Centrale (40 minutes) and change there to a regional train.

For the Lamborghini museum near Bologna, you change to a regional train heading to Funo Centergross (18 minutes) and walk 15 minutes to the museum (or take a taxi).

For the Museo Casa Enzo Ferrari in Modena, you take the fast Frecce train to Bologna Centrale (still 40 minutes) and change to a regional train heading to Modena (20 minutes) and walk 10 minutes to the museum.

In either case you should allow a minimum of 20 minutes to change trains at Bologna Centrale, as the station is huge and confusing, and you need to go from the deepest track level that serves the Frecce trains to the upper level for the regional train platforms.

If you mean the Ferrari Maranello museum, I would guess you take a taxi from the Modena train station. It is about 20 km away.

According to my sister probably the Ferrari museum. You can also stand outside the factory and watch the new cars drive out.

The Ferrari is more of a museum and Ferrari Experience.

Lamborghini is a visit to the factory.

They're relatively close to each other. I'd go to both if time permitted.

Rest assured that after visiting both (which is what I would do) the expert in which is better will be your son.

Since you will not be using a car, I would recommend the Ferrari Museum in Maranello. It is a museum that encompasses everything from road cars to race cars, helmets to trophies. About a 6 minute walk away is the Ristorante Cavallino. The restaurant that drivers have eaten at for years. The restaurant itself has historical artifacts within in. The factory itself is directly across the street from the restaurant.
The Fiorano circuit is also a short walk away. Since it is Italy, you can get right up to the fence and watch cars (if they are out).
It has been a while since I have been to the Lamborghini museum. When I went, it was a small museum. You did not have to take the factory tour (if one is available to you).
I ma sure your 8 year old will enjoy the day.

I would ask your son which brand he likes more.