Incidentally, if I don't have a good answer to a "guessing" problem immediately, I find it faster to just Google the relevant facts than to try to struggle to find a distinction between them that I can latch onto.

As for Hamburg vs. Cologne, my recognition heuristic is more familiar with Hamburg as a city than Cologne as a city (I know Hamburg is in Germany, I suspect that Cologne is in France). On the other hand, I know that I recognize Hamburg because I often eat hamburgers, which doesn't seem like it says much about the city. Nevertheless, if I have to guess, I'll guess Hamburg. Now to look up the actual answer...

Wikipedia gives 1.8 million for Hamburg, and Cologne (also in Germany, which surprised me) is slightly under 1 million. So I guessed right, but I still prefer the JFGI heuristic. ;)