Hi Charlottes,
Both cities are very interesting but I think Munich is much more affordable.
Vienna is quite expensive, and even though it's amazingly beautiful and full of places of cultural interest, I find it a "boring" city regarding nightlife and so on, whereas Munich is full of "biergartens", clubs and bars where you can have a good time after your German lessons, meet the locals, and enjoy a fantastic Bavarian beer!!
I hope this helps!