Spent 1 week in PRG last summer, followed by 4 nights in VIE.
Tough choice.
Prague looks better than Disney. It's a very old European city that managed to stay in tact during the war. Lots to do. I did a trip report on Prague last summer. If I can find it, I'll bump it.
Vienna is a more traditional European city. More like Paris.
If I had a choice I'd go back to PRG.