Venice or Naples as a side trip from Rome?

It sounds like you've already booked a round-trip to Rome? If not, you might try booking an "open jaw" ticket so you can fly into Venice out of Rome (or vice versa unless you would have a super early early flight out of Venice, which is sometimes a pain). That way you could start in Venice, spend a few nights, and head south to Rome and save a train trip and half a day of you trip.

(Don't search fares as two one-way tickets - make sure you book it as one ticket.)

If you've already booked a round-trip into/out of Rome, your other option might be to catch the train immediately to Venice as soon as you land in Rome. I did this on my first trip to Italy, and it worked out for me. My flight into Rome was really early anyway so I burned of that first jetlagged day on the train up to Venice and still got into Venice at a decent time. I preferred this to getting a hotel in Rome first, then changing and having to stay in Rome again at the end.

Or you could just wing it and if the weather is OK and you can get rooms in Venice (if not Carnival) just go for it.

I have been to Naples in early May and in October and it’s has been hot both times but definitely cooler in May. February, who knows? Rain is a possibility but the sites will be quieter and less crowded. I have been in Venice in March and found it cool and damp; but then it could be nice Spring weather, again who knows. I was quite chilled on occasion. Venice is one of my favourite places but I didn’t enjoy it as much in the dull weather.. it needs the sun!

You are sensible to take 2 nights in Naples. Do try and make it to the archaeological museum, that’s where all the artifacts from Pompeii ended up, what you will see at the site are replicas. At Herculaneum a lot of the mosaics and frescos remain in situ.

Hi, Sounds like a very fun trip with fewer tourists, unless you go to Venice during Carnival. Personally, I wouldn't go to Venice as winter in February can be quite cold & wet. While Naples & the ruins may have rain, I've seldom been as chilled by the damp cold as in Venice mid winter (December & January). In addition I'm not sure how the energy crisis will effect your trip, but my recollection was none of the churches were heated in winter. Brrrrrr. I would suggest leaving Venice for another time of year.
I agree with @Andrew H., try to search for open leg tickets so you don't have to take up so much time traveling. Even if you've already booked your trip, you may be able to alter it on-line. I loved Naples & a ferry trip to Capri on a sunny day would also be glorious. Lots more written elsewhere on travel to Naples & nearby ruins, you can search on RS home page, just type in Forum Naples etc.

Venice is too far from Rome for short side trips. The Naples area has much to offer with Naples, Pompeii, Herculaneum, Sorrento, Capri and the Amalfi Coast. It deserves at least 4 days. Still, if you want to do two nights there, that is fine, but why keep your hotel in Rome while there.
Do Naples at the beginning or end of your trip and don't waste paying for two hotels.

Friends went to Venice for Carnaval and it was not a good experience for them. Very, very crowded, rowdy.

Since you both want to see Pompeii and Herculaneum, consider spending a couple of nights in the town of Pompei. That way, you could see both sites, and add Oplontis and Stabiae to your Herculaneum day. Some people on this forum are dismissive of Pompei, but we stayed there in April 2022 and really enjoyed it.

http://pompeiisites.org/en/stabiae/

http://pompeiisites.org/en/oplontis/

https://www.askostours.com/

https://hotelforumpompeii.com/en/

Two very different experiences to choose from! I think with Venice, your traditional sightseeing will take a backseat to being part of a unique experience. Wether that’s a positive or negative depends on you. While Naples/Pompeii/Herculaneum are great, I would consider that regular “sightseeing.” Naples/Amalfi Coast is a great trip on its own and you might want to think further into the future about making that its own trip.

My only other note, I believe flooding in Venice is a distinct possibility at that time of year? That may factor into consideration this time around.

Venice Carnival dates are Feb 11-21 (shorter than in past years), so Venice shouldn't be crowded when you're there. I've been to Italy - from Milan to Venice to Rome to the Amalfi Coast over several visits in February and managed quite well with the weather. It's a great time to go, and you'll enjoy low season hotel rates and fewer tourists, though you still should book the most popular sights in Rome in advance. I doubt that the weather will be much different in one place or another. It's mostly just dumb luck. Take rain gear and plan indoor days and outdoor days and be flexible.

Venice is also one of my 2-3 favorite cities in the world. But Naples is so much closer and so interesting. Pompeii is better on a sunny day because some of the walking is difficult on the ancient cobbled streets. I ended up in Herculaneum on a rainy day and still enjoyed it - there were buildings you could enter when the rain started to come down heavily. There's much less walking and it's easier. Be sure to include at least 2 hours in the National Archaeology Museum in Naples. The RS guide has self-guided walking tours of some sights and a good city tour of Naples. (Also lots about Rome too).

Since it's likely that your next visit will also be in February, do consider spending time in Venice during Carnival. It's an amazing experience and the crowds are mostly avoidable.

While I'd generally recommend spending all of your time in Rome in one stay, for Naples I like the idea of going in the middle of your trip.

The Circumvesuviana train makes it unnecessary to stay in the banal modern city of Pompeii. It's probably harder to book a hotel in Venice, but it's much more magical (to say the least) than Naples. Have you looked at climate sites for temperatures and rainfall in February?

It's probably easier to book Naples than Sorrento (even in February), but Naples can be a bit gritty for people who don't like large, busy cities with fierce auto traffic. Since you have been to Rome before, I assume you don't think that Naples and Venice are suburbs of Rome, suitable for one night out. They are easy trips, but there's overhead involved in changing hotels and learning current local transportation. Any chance you can fly home from Venice?

I would guess that both places have less attractive weather in Februrary. I personally would go to Venice, but Pompeii is unmissable.