I would not bother with the flat rate places. You'll be expected to buy very expensive drinks, and the girls are rarely that into it.
Of the brothels, I think Royal is the best of the 'high class' ones. You can find seedier ones, but I wouldn't bother.
Artemis is well worth a visit, if you're prepared to stay for several hours, and make the most of it, but I think you need to be prepared to see three women to get your best value from it.
I would not bother with the street scene - it's very hit and miss.
Finally, I would absolutely recommend Erlebniswohnung. Gangbang/sex party every night. Great value, and 4-6 women on at a time, with probably double that of men. Great vibe, decent food, sauna, showers. I've spent many happy evenings here. Very highly recommended: http://www.erlebniswohnung.com/index.php/en