I'd say Budapest, and I'd vote for taking the train here from Vienna as opposed to driving (arriving in BUD on the highway can be pretty confusing and you don't need the car once you're here) or hydrofoil (cool, but you're stuck inside the boat the whole way - no on deck experience). While it's not a real 'castle' the Castle District in BUD is fun, and I think it'd be interesting to compare both cities of BUD and VIE as they have been so closely linked for many years.