We are planning to stay 4 days in Sorrento and originally planned a long day of Pompeii and Naples. But, would it be better to tour Pompeii one day and Naples the next? Is that too much train travel? Opinions?
Pompeii is about 45 min. from Sorrento, and from there to Naples is another 40 min.
I would probably spend a day in Naples, as there is so much to see there.
Circumvesuvian timetable http://www.eavsrl.it/web/content/orario-ferroviario.
Since you're visiting The Amalfi coast and Naples, look @ http://www.campaniartecard.it/
The one you would want is the Tutte la Regione 3 day card as this includes transportation.
Could save some money.
1 day Pompei. Next day Naples. It would be better to stay at least one overnight there. Circumvesuviana train is usually hot and crowded. Much better option is to go by boat from Sorrento to Naples (and back).
I would do one day in each.