Laura, point-to-point may be slightly less expensive. As a youth, you can get the 5 day/2 month Eurail pass for $219. The pass does not cover reservation and supplemental fees which could add 30% to this cost.
For Point-to-Point, Milan to the CT is 21.30Euro ($32). The CT to Florence is 15.10Euro ($23). Florence to Siena is 10Euro ($15). Siena to Naples is 34.90Euro ($53) plus the fare on the Circumvesuviana from Naples to Sorrento. Naples back to Rome is 19.50Euro ($30) plus the Circumvesuviana fare to Naples from Sorrento.
Aside from daily town-to-town trains in the CT, for which you should not waste your 5-day pass and costs on the Circumvesuviana train from Naples to Sorrento and back, your looking at about $153. This is based on 2nd class standard fare. If you can buy your tickets no later than the day before travel, you could get the "Amica" fares which are 20% lower. That's about $123. Point-to-point is better.