I would go in Europe summer - I much prefer warm weather and even summer is not hot compared to what I am used to at home
We went in late September/Oct , (similar 14 day trip with Scenic) was bit cheaper then compared to July/August, weather was still quite warm, short sleeves every day, we were lucky we had no issues with water levels and the only place that was crowded was Rudesheim, they had some festival on which ballooned the crowds