Antwerp/Ghent in a Day?

Personal opinion question, but I preferred Ghent to Antwerp and both over Brussels. If I was going to stay in one rather than return to Brussels, I’d choose Ghent since it’s between Bruges and Antwerp. In all fairness, we stayed two nights in each and enjoyed both.

Only you can answer your question correctly, as we have no idea what your interests are or which spots you'd like to visit in each city. Even the date you plan this visit can have an impact depending on whether some important event is taking place.
Let me also mention that reaching Ypres (by public transport) is easier and more practical from Ghent than from Bruges.

I can understand the preference for cute, smaller cities over Brussels purely for walking around; the Grand Place in Brussels is beautiful but tourist-swamped. However, I would be remiss if I didn't report that I willingly spent 13 nights in Brussels earlier this year. It is rich in Art Nouveau architecture, which I love, and has some good museums, despite the current closure of two parts of the Royal Art Museums.

I haven't gotten to Antwerp yet, so I don't have an informed opinion about it. However, I wouldn't choose to split a day, or even 1-1/2 days, between Ghent and Antwerp.

I too love Brussels. The problem with Brussels is that it takes longer to give up its charms. If you are just the casual tourist, there for half a day, you see only the Grand' Place (which is stunning by the way), the peeing boy and all the tourist crowds. But if you give it a bit longer - say 3 - 4 days - then you have time for the museums, the architecture; the 44 tram from Montgomery to Tervuren is Brussels' prettiest tram ride and you can visit the Park de Tervuren and the Museum of Central Africa at the end of it. Brussels is also a lifestyle city and by that I mean food is very important. Take the time to go to some of the better restaurants like those in the Galeries Royales de St.-Hubert or tarry a while in the bars of the Parvis de St. Gilles or Place du Luxembourg. I also like markets and Brussels has many of those; the Saturday antiques market in the Sablon is just lovely. Stay somewhere nice like Le Dixseptième on in the Sablon. Go shopping in the Louise district. Cocktails on the rooftop of The Hotel, which has a view. Brussels has its kinder side, you just have to look for it.

Lavandula

Antwerp also has a respectable amount of Art Nouveau and Art Deco architecture. It is smaller than Brussels but far larger than the other named cities. It also has a large number of diverse museums, including the world class KMSKA, home to dozens of important Rubens paintings.

My wife and I spent five days Antwerp and we could have spent more time there. This includes a day trip to Mechelen - an incredibly wel-preserved town. There are lot of other things to see, especially if you like museums. Just keep in mind Antwerp is a big city. We found public transportation challenging. Signs are in Dutch and there are not many maps in the system. The good news is the Antwerp City Pass includes public transportation, except the Waterbus. We took the Waterbus to Lillo (an hour from Het Steen to Lillo and an hour to get back) enjoyable.

The Antwerp City Pass is a waste of money for most of the visitors. All public transportation in Antwerp can be used by swiping your card (Debet or Credit with Mastercard logo), including the waterbus. Only you can make your calculation with the individual ticket prices of museums you want to visit

No. You need at least a whole day for each of Antwerp and Brussels, or two days for both, if you are skipping the museums. If you are seeing museums, you need one or more extra days. You need a written itinerary listing what you would like to see in approximate order of importance, along with the supposed opening and closing times. The 4 major cities in Flanders in northern Belgium need about 7 to ten nights if you are seeing more than a few sites or museums. How many nights total do you have in Belgium? You are rushing your trip too much or skipping a lot.

Edit: it is fine to skip Antwerp and Ghent if you are not going to give yourself enough time in Belgium and if you would prefer to focus on Brussels, and Bruges and Ypres and battlefields. I spent 3 nights in Antwerp and two in Brussels. Unfortunately I didn't make time for Bruges. Antwerp has some good museums and I have a talent for seeing underappreciated museums that a lot of "drive by" tourists skip. You don't have to see every museum and monument and you don't have to rush through a few streets of the historic centers of Antwerp and Ghent just to say you were there. Don't go to the historic center of any town if you are going to be in a darn hurry to rush off to another town later that day.