Getting from Venice to Naples

For the most part, the trains take 5h10min or 5h30min (the latter trains require a change, often in Bologna) from station to station. The full base price for Standard/2nd Class is a little over €100. If you buy ahead a discounted ticket on line (Economy or Supereconomy) you can save substantially (up to 40%) however consider that discounted tickets are non refundable if you miss that train, therefore you must be certain of your travel time.
Sales online start 120 days prior to departure and discounted tickets go pretty fast.
Flying is just over 1 hr, however you need to add:
- Transfer from Venice to Venice airport (at least 45 min)
- Check in/Security Procedures (75 min)
- Deplaning/Baggage Claim in Naples (20 min)
- Taxi from NAP to city center (20 min without traffic, which in Naples applies only in the middle of the night).
So flying might save you one hour at most, although it requires extra hassle compared to sitting on a train sipping wine or reading for 5 hours. Trains are also infinitely more comfortable than planes.
The cost of the flight might be cheaper, but then once again you need to add the cost of the transfer from/to the city to/from the airport by taxi or shuttle (or boat, in Venice). The train departs from the city center and arrives to the city center, therefore those costs are lower or none.
AirOne and EasyJet are the only 2 airlines flying from VCE to NAP non stop.
You decide.