I like both cities, though I know Berlin better.
If you're interested in 20th Century history, there is no better place than Berlin and since the Fall of the Wall, much of it, especially what was East Berlin, has been rebuilt.
Hamburg is a bit closer and the people are very friendly. The City Hall has six more rooms than Buckingham Palace. And we had a great time when we went there for a friendly pre-season 2003/4,
And you will certainly be able to buy any one of a number of excellent beers in both places.
So, it's down to personal choice, IMHO.