Ostia Antica or Hadrian's Villa

They are not comparable, and they don't have the same amount of local travel-overhead. Sometimes, people compare Ostia Antica with Pompeii, especially when considering the (spurious ...) idea of doing Pompeii as a runout from a bed in Rome. Indeed, your time schedule is likely to be the deciding factor. I consider Villa d'Este to be a more important visit than Hadrian's, in Tivoli. We went back to Tivoli twenty years later to see Hadrian's Villa. Both can be done independently by public transportation, but it takes planning.

https://community.ricksteves.com/travel-forum/italy/tivoli-day-trip-from-rome

Although I don't agree with those who say that Herculaneum is better than Pompeii because it feels less "destroyed", I would observe that Hadrian's is less "destroyed" than Ostia Antica. But it's a private, executive, villa, not a city.

Another question is what your plans are for either lunch or a respite from sun and heat. I have not been to Ostia myself, but I will say that Hadrian's has more shade than many large classical ruins. Also, Hadrian's is much more often "cited" in books, art, architecture, and theater designs than is Ostia.

When taking RS South of Italy tour we spent a couple of hours at Hadrian’s Villa - not enough time for the size and complexity of the site. Very impressive ruins; I could have spent the whole day there. Did not see a cafe at the site so you would need to take a lunch or eat in Tivoli. The site is not easy to get to by public transport, maybe a car/driver would be the best option

I preferred Ostia Antica. Easy to reach by metro/train from Rome. A whole city. Easy walk-in as it is on a floodplain. The city was abandoned due to silting up of the river so does not have the tragic story of Pompeii, which is why I think it remains a relatively unvisited site. It has a theatre, a forum, temples, a necropolis, a forum, a public latrine, shops, fast food joints, bakeries, etc. I spent a day there - it was not long enough! There is a small museum/gift shop and a cafe that offered a decent lunch, sandwiches, pasta, salad. Also it was still being excavated when I was there, so you may see some archaeologists at work. If you get bored you can head to the beach at Ostia, another few minutes on the train, or head back to Rome.

I am in Tim's camp and agree totally. Simply not the same. OS is very shaded as it is in a park that is heavily wooded. And access to OA is much, much easier.

We ended up at OA on an overcast Sunday morning in May. It was nearly deserted. We're not fond of tours and poked around to our hearts' content. It was really enjoyable and the train ride there charming. Who knew the Italian kids would enjoy the accordionist who hopped on and off at different stops so much?