Heathrow? By far more expensive....

It depends what you are looking for and from what Terminal you are leaving. For alcohol and cosmetics it does not really make a difference, I believe the big tax free shops are both operated by the same chain.

If it comes to other products, Frankfurt might offer more. However, I hardly see any advantage compared to downtown prices.

The attitude that DUS iis geared primarily towards Russians and Turks who constitute the vast majority of non-EU travelers at the airport is certainly incorrect. There are not many non-EU flights ex DUS, however, the vast majority of non EU flyers should go to NA with three A340 departures.