Via Cassini/Caserta was good the time I did it, +10 years ago, probably better than the high speed line. I would personally recommend the route.
I suspect that’s neither of your choices above!
You see the same scenery via Cassino and Caserta as you do from the high-speed line as they follow each other closely along the same valleys for most of the way....although of course you'll see it much more fleetingly and in less detail from the HSL. The IC route via Formia probably has a greater variety of scenery....starting off with the Roman acquaduct in the Southern outskirts of Rome....then crossing the flat, intensively-cultivated reclaimed Pontine Marshes to Latina and then tunnelling through several high, rugged limestone mountain ranges at right angles to the coast. After the stop at Formia, there are good sea views of the Gulf of Gaeta, and then on the coastal plain between Minturno and Villa Literno you will pass numerous water buffalo farms, whose milk is used to make the famous Mozzarella cheese. After some pretty grim suburbs around Aversa - the only place in Italywhere I have ever been in a train which has been stoned - the final run into Napoli has excellent views of Vesuvius on the left hand side (as do the other two routes).
If you have time to spare in Napoli, I can recommend a visit to the excellent Italian National Railway Museum adjacent to the suburban station at Pietrarsa-San Giorgio a Cremano, on the line towards Salerno. Ideally, spend a day there, with a break for lunch at a local restaurant....but if you only have half-a-day, you should be able to get round most of it.
