You choose well with Vienna and you choose well with the region in general for the holidays.
Definitely do see a concert in Vienna as there are some excellent world class music venues in Vienna. Vienna on the whole may bring in performance companies that are maybe a little better than Budapest or Prague. I don't have such a discerning ear that I can tell, but if someone were to tell me this I would be inclined to believe it. After all, there are more tourists "of a type" with more money than you will find in Budapest or Prague. But something you will get in Budapest that I found lacking on my few trips to Vienna and Prague is a feeling of being part of something unique to the City. I think because in all three of the performance I saw in Prague and Vienna we found ourselves surrounded almost entirely by tourists...........and we bought some pretty good seats.
What we have enjoyed in Budapest is that the Opera House and the other venues are still very much a local affair and not yet formatted to please tourists. In the dozen or so performance we have been to in at least four venues tourists have represented a small minority, and everyone has been a sellout or a very near sellout.
For a totally different experience book a performance at the Hungarian State Opera (as grand as any opera house in the world),
http://www.worldwideticketing.com/images/opera-ballet/hungarian-state-opera/hall.jpg Where one of those lovely boxes can be had for about $75 a seat and some really grand seat go for as little as $45. You will find us there on New Year's Eve.
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The Budapest Operett Theater where the productions incredibly well done, many with English subtitles, many where language isnt an issue at all: http://dancsmarket.com/wp-content/gallery/hirek/finale.jpg
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The Palace of Arts in all its modern splendor brings some of the finest production companies in Europe: http://www.budapestagent.com/wp-content/uploads/2014/03/mupa4.jpg
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The recently restored Grand Room of the Liszt Academy (where you can find us a couple days after Christmas): http://www.resmusica.com/wp-content/uploads/2013/10/academie-franz-liszt.jpg
In Budapest, if you should decide to give it a try, purchase the best seats you can afford. The absolute best are only abut $70 and at the Opera House this will give you access to the club and the balcony at intermission at the Operett it will get you access to a private lounge; both provide you the opportunity to meet and interact with Hungarians. We have had some great conversations at intermissions. Google the images of the venues, look at the high resolution pictures and decide what to wear.