Milan

I'm a train guy. Trains don't get delayed by rush hour traffic, but it all depends on the circumstances. If you are arriving on another train at Centrale, makes more sense to just change platforms than go outside and locate the bus stop. If you are in a local hotel, the bus could make more sense. The fastest way is the train from Cadorna station, which is a short walk from the Piazza Duomo and on the Metro green line. It costs a euro more, but is nonstop for the most part. The Centrale train has to back out of town and come back in to pick up passengers at Porta Garibaldi station and then makes a couple of intermediate stops on the way to Malpensa.
It can also make sense to take the bus depending on which terminal at Malpensa you are leaving from.

I am flying on Delta out of terminal 1 and I am staying in an hotel very near the Central station. (Milan)

Personally I would take the train. The bus leaves from the south east corner of the train station so access is not a concern but the train is more comfortable. Either one works pretty well.

Thanks for your help!!