I hate when someone asks me that
. The problem is that we have a real bias against any organized group tour and its difficult to overcome ones biases. The reality is that the answer to your question depends on your level of comfort regarding doing things on your own. The advantage to the cruise line tour is that it takes you from pier to site to pier with usually a stop at some phoney jewelry operation or cameo factory where the tour company and cruise line get a kick-back on any purchases (do you think I am cynical). Going on your own you will need to get to the Naples train station, buy your ticket, validate your ticket, find the right track and train, etc. The main advantage to going on your own is that you make the decisions on how long to stay at a place and you save money not to mention you get to choose your traveling companions. Ok, I am getting off track. Pompeii is relatively easy to do on your own (admission is about 12 Euros) but if the thought of getting to the Naples station and taking a train puts fear into your gut you should take the tour. If you have the spirit of adventure, some "travel smarts," and like making your own decisions than go off on your own. Whether you take the tour or go on your own, Pompeii will only fill half your day so you still have decisions regarding the remainder of the day.