If everything else is equal, I personally would choose Munich instead of any of the other 3 heavily populated, industrial northern cities you mentioned.

No matter whether you pick DUS or MUC though, my advice would be to get there and then get out of town to the countryside. Somewhere. Anywhere but in the big city! Buy a cheap train ticket and head for a much smaller town for a more relaxing and enjoyable experience. Also it will probably be more to your budget-conscious liking!