Well, it is very unlikely that you will get a job in Germany in 2-3 years. In Eurowings these position will be taken by the GWI guys and even if, you would be the last in line. Other option GMI??? Not really if you don't wanna fly for EWE. So I would take into account that you won't get back to Germany very soon.
Anyway, Vienna is a great place to live.
If you get the chance to stay with OS longer than the 2 years, take that job. The Dash is the best a/c for you to learn to fly, the A320 is a very good rating to have, but you learn how to setup the FMS. OS is a network carrier, you will get the chance to fly jet sooner or later, EMB and maybe in a few years even long haul on a 767. OS is short of pilots, so there will be quite a lot of movement. Have a look at the lastest agreement.
So if you are planning a long term career, take the Dash, if you want to move on quickly, like the middle east, take the 320. Anyway, chances to come back to Germany are very low, so don't focus too much on that.
BTW. I have flown Dash-8 and A320 and I lived in Vienna, so know what I am talking about.