Budapest or Bratislava?

If you mean literally, a day trip, then Bratislava must be your choice. It is a perfect one-day destination.
Budapest, in my opinion can be largely appreciated in two days but most would suggest a bit more time.

Bratislava old town exists for tourism and tourism alone. But it's sort of quaint for the looks and style. It's close so you can spend a lot of time efficiently. But I understand reaching it from Vienna is a tiny bit difficult at present. Not the sort of place i enjoy ... too much like DidneyLand Main Street. But others love it, so do some research and see if it appeals to you.

Budapest is 2.5 hours away, then figure 20 min more from the train station to where you can enjoy and see the sights for maybe 6 hours before going back. And you could see quite a bit in those 6 hours. I could map out a route for you (some of Andrassy ut, Great Synagogue, Great Market Hall, Danube Corso. Parliament, Basilica, lots of views and great petiof neighborhoods ... at a minimum). Better if you stayed the night on the river and caught an 8am train the next day, or arrived late stayed the night and then took a late train the next day.

Other ideas are Melk, Gyor / Pannonhalma Archabbey, Sopron. Really about what interests you in topic, time and effort.

Favorite Tourism Video of Budapest: https://youtu.be/1nd5AtZIrTk?feature=shared
Anthony Bourdain describing how visually pleasurable Budapest is: https://youtu.be/0nd9DuDGCz0?si=aa0c19KaJHEtabkX
Favorite Guide Book because its small and does the essentials: DK Eyewitness Budapest Top 10 https://a.co/d/6JD82mf

Thanks to both of you for the info!

My sister and I are headed to Vienna (and Prague) in March/April. I always like to have several day trip options because you just never know how much time you’ll have. I keep finding more things for us to do in Vienna, so we might end up not taking any day trips. But, these are my options:

1) Budapest: This was originally my top option, but it has, unfortunately, fallen to the bottom of my list. I just think it’s going to be too much for us. The 2 hour 39 minute train ride is not terrible. Trains are comfortable and they have bathrooms. But the getting to and from the train stations adds time. We’d have to get from our hotel to the Vienna strain station. And it’s not like you show up at 6 am for a 6 am train. You have to allow yourself time at the train stain. Maybe you want to buy a snack or use the bathroom. And then you need to allow enough time to find the train platform in an unfamiliar station. And when you get to Budapest, the things we’d like to see are not within walking distance of the train station. So, again, this will add some time. The more I thought about it, I think it’s just going to be too much travel time. But, I am keeping it on my list, just in case. If we’re feeling great, maybe we’ll go for it.

2) Bratislava: This is also on my list, but as Mr E mentioned, slightly difficult to get to because of some construction work. You can read more about that here:

https://community.ricksteves.com/travel-forum/austria/fyi-day-trip-from-vienna-to-bratislava

3) Sopron, Hungary: This place I saw mentioned on the forum and added it to my list after watching this video:

https://www.youtube.com/watch?v=ofKPhBt_8Qg

The train ride is 1 hour, 6 minutes from Vienna Meidling station. The old town is within walking distance of the train station.

4) Melk: This is my final option. We’d have to do this one at the end of our trip because the park/garden does not open until April. You can read about the Abbey here:

https://www.stiftmelk.at/en/

I have in my notes that this is 1 hour by train. Depending on the time of your trip, there may also be an option to take the boat.

I hope that helps. Have a great trip!!

Bratislava/Slovakia Prime Minister, Robert Fico made it clear he is aligned with Putin's goals in Ukraine. For that reason we are bypassing Bratislava/Slovakia completely on our 4 month trip thru Europe this spring.

I'd go with Budapest.

And when you get to Budapest, the things we’d like to see are not
within walking distance of the train station. So, again, this will add
some time.

Understood. Then for the next person that reads this. With 6 hours and a day pass for the public transportation you can see quite a bit with out rushing around if you plan it out well. Happy to help with that. Fortunately the train station is served by a metro line that runs direct to the center of everything, then the trams (like HOHO tour busses) take over and whisk you to the sights. Most everything is within 100m of a tram stop.

I did work out a day trip tour for Budapest if anyone is interested, PM me.

I think Bratislava is your best bet. It’s so close to Vienna and allows you more time to visit. It’s a pretty small city and you will be satisfied with just a day trip. While Budapest, you will certainly want to spend more time and half the beauty is seeing its iconic buildings light up along the river at night. If you do choose Bratislava, I quite enjoying going a bit out of center and visiting the Devin castle ruins.

Christina, I don’t disagree with that at all. But everyone is a little different. I think the pivot point is in the nature of the day trip. If one were to depart Vienna about 8am then stay the night in Budapest and take a 8am train back to Vienna, then it would be hard for me to say Bratislava makes more sense from my interests and point of view.

But doing a long day where you show up at 10:30 and depart at 20:00 can be fun if you just gotta see it. I’ve visited a few cities like that …. When I was younger …. Then returned to the ones that I liked. But I like I said, I was younger. Not what I would do today.