Both places are worthwhile daytrips from Rome
Both Ostia Antica and Pompeii offer unique insights into ancient Roman life, and both places are worthwhile day trips from Rome. Unfortunately it would be difficult to visit both of these in one day. Some people prefer to go to Ostia Antica as there is usually less people and it is a lot cooler during the Italian summers, and although Ostia Antica is very accessible from Rome, Pompeii is one of the most significant and well-preserved examples of ancient Roman history.
Pompeii is far more significant than Ostia Antica
Pompeii has a more interesting back story
Pompeii was covered in volcanic ash from the towering Mount Vesuvius in the year 79 A.D. The city was covered in a way that preserved the city in a way that gives incredible insights into the way people lived during this time period.
The excavations of Pompeii are remarkably well preserved
Pompeii is the only place where you can have such in depth insights into ancient art, housing, customs and trading, and it is a truly magnificent and striking place. The excavated area still holds evidence of furniture and household trinkets that people during the time would have had, shops and workshops where people worked, and in some places you can see people that were covered in ash and preserved at the time of the eruption. This is a fascinating archaeological site and offers the best view of ancient history.
You can combine Naples and Pompeii in a day
If you are taking a day trip from Rome to Pompeii, you can also see the city of Naples, whose city centre is an UNESCO World Heritage site. Naples is a great city that many visitors skip - it is the home of Pizza and the Archaeological museum contains most of the relics form nearby Pompeii.
The ruins at Ostia Antica are closer to Rome, and you can experience the architecture up closer than you can in Pompeii, but Pompeii truly is a unique place to visit if you have the time to do so in your itinerary.