My initial reaction when reading the thread title was "Obviously HK, you fool! What kind of question is that?"
But then it turned out that you are married, pussy is not a factor, you already have some foundation in German and come to study.
So let's take a closer look. I lived in both cities, so I feel like I can compare them somewhat. I'll focus on Vienna though.
First and foremost,
security:
Not an issue, both cities are extremely safe. That's what I would have told you five years ago.
Unfortunately in Vienna the situation has changed a bit, something you most likely won't notice as a visitor, but have to take into consideration as a resident.
Regarding the migrant situation, nobody knows yet how bad it will be.
We'll have to wait for the summer and see, the migrants have made previous obersvations and experience concerning the safety in Vienna null and void.
Just imagine the current situation as the calm before the storm. The shelling is coming closer and closer.
In the gentrified, central area I used to live in, popular with tourists, there have been three murders/stabbings in broad daylight, two rapes, and two attempted rapes since the beginning of this year alone, all involving migrants. A few weeks ago there was an extremely violent clash between Afghan and Chechen youth gangs next to a popular shopping center, almost 100 people involved, again in broad daylight. They are fighting for control of the drug trade I heard, against the Africans and various Balkan gangs.
Also, not too long ago a person I know whas cycling through the city on a nice sunny day, when a Serbian opened fire on another one. Too bad he didn't aim very well and shot a young Austrian boy passing by, survived luckily. In another district there were two shootings recently, one in a tramway. I think it was Serbians against Albanians. Austrian girl got the bullet, survived luckily. Not to mention the car bomb, the brutal rape of a child in a public swimming pool by a migrant, harassment of women by the same group in others, and various other incidents.
Too many really, those were just the ones I remember right now.
Now, I don't want to engage in too much fear mongering. It's just my observations from the ground, naturally with a bias.
Vienna is still a lot safer than Paris, London, or Brussels. Probably also safer than almost every large city in the US.
Point is, all of those things I mentioned would have been pretty much unimaginable ten, even five years ago. A young girl could have walked through the worst part of the city in the middle of the night and it would have been perfectly save. Not anymore. Times are changing, and there is a clear trend.
In comparison, from what I read and hear, Hong Kong is still as safe as it used to be.
Quality of life:
You will probably know that Vienna topped all the important global rankings in this category for the last few years. Keep in mind though that they are usually meant as advice to MNCs, with the lifestyle and bankroll of a typical expat as standard.
Nevertheless it's still great. Vienna has a fantastic location in the heart of Europe, Laner elaborated on that. Public transportation is world class and very affordable in Vienna, you won't need a car at all. Quality of food is good, restaurants are plentiful, with a large variety of cuisines, and affordable. The price level for groceries is higher than in Germany, especially for hygiene products. I think coming from the US you would definitely notice, this is due to an oligopoly in this sector.
Austrians are fiercely opposed to GMOs, there's lots of organic and regional stuff available. Tap water is of higher quality than most brands of bottled water, directly from the Alps.
The weather in Vienna is ok. Winters and autumns suck big times, just cold without snow nowadays, very windy, grey and depressing.
Spring is alright, summers are nice, the heat is mitigated by the constant wind and the river Danube.
The rental market is in a bubble and overheated right now. Newly developed real estate is catered towards young professionals and DINKs. Affordable accommodation for students and families is becoming very rare. Expect to invest some time into searching for a place there, especially long term.
Internet speed is alright, not as well developed as the rest of the infrastructure though.
Honestly too lazy to write about HK now.
Short version: It's hot, it's humid, crowded and (except the NT) relatively overpopulated, value for money when it comes to renting a place is terrible, food is awesome, quality of groceries is mediocre, general price level is high for Asia, public transport just ok, and it's a great transport hub with cheap flights to the rest of East and SE Asia.
Girls and Game:
You are married.
Language:
In Vienna, you'll be fine with English. Given that you already have a basic understanding of German, it's just a matter of learning your vocab and practice. The Austrian dialect might be confusing when you start out, but it's not really prevalent in Vienna anymore and everyone is capable of speaking Standard German. The media is in Standard German.
In Hong Kong, the dominant language (not a dialect) is still Cantonese, which is mainly spoken in Guangdong province. It is very different and not mutually intelligible with Putonghua/Mandarin. In my opinion, forget about learning Standard Chinese there. You'll lack the immersion necessary to really get a good grasp of the language, and also the writing system is different from the PRC, with traditional characters used instead of simplified ones. Cantonese is even more difficult to learn than Mandarin, and less useful.
English is spoken, but not as much as it used to be. I'd even say it's more useful in Vienna nowadays.
Business/Networking:
Hong Kong, Hong Kong, and Hong Kong again.
Not even close.
Situated in the Pearl River delta, with manufacturing power house Guangdong next door, the megacities Shenzhen and Guangzhou a convenient train ride away, three of the world's top 10 container ports, and HK is a global finance hub and true world city too.
Trust me, with your options, HK is where you wanna be.
Now about Vienna, there's something you need to know. Austria invented and practiced crony capitalism before it was cool. The whole country, economy, bureaucracy, society, is divided amongst the two ruling parties, the People's Party and the Socialdemocrats, as determined by the constitution.
You don't get a job via qualification and merits, you get it via connections, by knowing the right people, maybe friends and family.
Often the same for promotions.
The Balkan starts in Austria. This is not Germany.
Three words that absolutely, 100%, don't describe Austria or Vienna:
Dynamic, innovative, vibrant.
Your chances of getting a job there as a non-German speaker without a substantial network are very low. Maybe as an intern in some international organization.
Visa:
Shouldn't be a problem in Hong Kong I'd say.
Austria is part of the EU and the Schengen Agreement.
The country is known to be very strict, compared to other EU states, when it comes to visa applications. You will not automatically get a working visa after finishing your MBA, this I can tell you. Again, I don't really know how difficult it is for US citizens. But I can tell you what you'll need to apply for a student visa:
1.)
You'll have to prove that you won't be a burden to the Austrian state. To do this, you'll have to show proof that you own at least 10593,36€ as a single person, or 15882,96€ as a married couple. You'll have to do this once every year to extend your visa.
2.)
You'll have to have health insurance valid in Austria.
3.)
You'll have to show proof of accommodation, e.g. a rental contract.
4.)
You mustn't pose a threat to public security.
5.)
Passport
Birth Certificate
Marriage certificate if required
6.)
You must be accepted by an Austrian university.
All documents may be required to be translated to German.
As for your wife, her visa type depends on what she intends to do in Austria.
All in all I'd say Vienna is a great place to raise a family or to retire.
Otherwise the city itself is dead, nothing happens there. It's the world's largest open-air museum.
A city for old people.
If you have nothing left to prove, if you've made it, if you've found inner peace and are satisfied with where you are in life and what you've achieved, then move to Vienna. You'll be happy.
If you are young and hungry, up-and-coming, ready to conquer the world, then don't.
There's other places for that.
Like Hong Kong.
If you have any further questions, feel free to ask or send me a PM.