Train or Air from Milan to Naples?

Train by all means. Just considering the time to get to the airport, check bags, go through security, etc, you could be 2 hrs into the 5 hr train ride. The trains are fabulous, comfortable and its relatively easy to navigate the stations in Europe, that taking one is always our choice unless we need to have a car. Cost is generally cheaper or comparable to flying for a far better experience. Italotreno.com or trenitalia.com

If you’re already at MXP, arrival, it makes sense to fly.

Ryan air, Easy Jet and Wizz air (budget airlines) have schedules that would fit most arrivals.
Flight time is 1H30m. Fastest train is around 4H 43m. Edit cost around € 30.

If you’re at lake Como or Milan it makes more sense to use the train.

If you are at the airport because that is your arrival point, continuing on by plane may be the better choice.

As noted above the train probably is better if you will have been in Milan for a few days and are then going to Sorrento, via Naples, the train is better. Getting to Sorrento from the Naples train station has more readily available options than the airport.

Are you sure you want to go to Sorrento in mid-August? It will be quite hot.

I agree with Gerri and Joe. If you’re arriving in Milan by plane, you can fly nonstop on Ryanair for $20. If not, there are direct trains from the Milano Centrale station to Naples.
Don't expect great views along the way. Train tracks don't run along the most scenic routes. Although there will be occasional nice views.

Take the train. It is 4:30 by direct trains, once you add check in, security and other airport faff flying will probably not save much (if any) time. It will also be by far a more comfortable trip, and with a lower carbon footprint.

Don't expect any amazing views, most of the trip will be on new high speed lines. Although you will get to see at least a glimpse of the cities the train stops in. And travelling along a motorway going three times the speed of the cars is fun, I can't deny that :-)

Agree with everyone here that if you are already at airport - fly. But Badger makes a good point. Train is a better way to go if not, and much more relaxing.