If you've learned German and you're choosing between Germany and Austria — or moving between them — the language is close enough that you'll manage either and different enough to be caught out. Most of what surprises people isn't vocabulary. It's register and convention.
The language differences that actually come up
Everyday vocabulary. Austrian German has its own standard words for common things, particularly food, months and administrative terms. These aren't slang or dialect — they're the standard Austrian forms, used in writing and official contexts.
Greetings. Grüß Gott is standard across much of Austria and southern Germany and sounds unremarkable there, while Guten Tag is the northern default. Servus works as both hello and goodbye in Austria and reads differently further north.
Dialect depth. Austrian dialects can be substantially harder for a learner than standard German, and the same is true in Bavaria, Switzerland and parts of northern Germany. This is a learner's problem everywhere in the German-speaking world, not an Austrian peculiarity — but if your German came from a Hochdeutsch course, expect a gap.
Ask a teacher who knows both to point out the specific forms you'll meet, because the lists in textbooks tend to be out of date.
Titles and formality
The difference people notice most.
Austria uses academic and professional titles more readily than Germany does — in address, in correspondence, on doorbells. Being introduced with a title and addressing someone by theirs is ordinary rather than stiff.
Germany is somewhat less formal about this on average, and in parts of the technology and startup sector considerably less so. But the north-south variation inside Germany is wide, and a Munich office may run closer to Vienna than to Hamburg.
The safe approach either way: start formal, use whatever the other person uses, and don't move to du before you're invited.
Workplace convention
Generalisations here are worth less than people pretend, and the variation within each country is bigger than the difference between them. An international company in Vienna resembles an international company in Frankfurt far more than it resembles a family firm in rural Austria.
What you can reasonably expect in both: punctuality taken seriously, directness that can read as blunt if you're not used to it, and a clearer separation between work and private life than in North America.
Paperwork and recognition
Not a language matter, but it's what actually determines whether you can work.
Professional qualification recognition is handled separately by each country, and a credential accepted in one is not automatically accepted in the other. The same applies to residence processes and any language requirement attached to them.
Check the specific rules for your profession and nationality in the specific country. This is the part where general advice is worthless.
Which to prepare for
Learn standard German, then adjust locally. Someone with solid B1 adapts to Austrian vocabulary in a few weeks; nobody learns Austrian German instead of German and comes out ahead.