I'd choose the Passau-to-Vienna trip. While we had a good time this year, it seems like we had more fun on the Passau-Vienna section. The trail was in better shape more consistently, stayed closer to the river, was a bit more scenic (except for the Danube Bend section of Vienna-Budapest) and there were more places to stop for snacks and hotels and to see the sights.

We had some challenges but didn't have huge problems with the language barriers on either trip and were lucky with the weather both times.

If you have specific questions about either section, I'd be happy to ramble on about them.