Because a German dunkel is not as well known as an English porter, I'm going to compare them side by side for you.
First of all, both are dark beers (the word "dunkel" means "dark" in German). I'm sure you know that the dark color in beer comes from the roasted malts used to brew them. A dark roast is given to both the dunkel and the porter malts, a fact which causes some people to think that they are similar beer styles. But the similar color is about where the similarities stop.
The first big difference lies in the roasting process used. English porters and stouts typically use heavily roasted "chocolate" malts that result in a toast-flavored beer. A German dunkel, like the excellent one from Göller, uses instead a Munich Malt.
The Munich Malt (Münchenermalz) is cooked at a lower temperature and for a longer time. The usual roast temperature for Munich Malt is between 120-160° F, then it's raised slowly to about 240°F at the very end. The result is a browning effect, but not burnt or toasted. Munich Malts give out more caramel notes rather than the charcoal notes of the kind you will find in a chocolate malt.
Basically Germans look for a Maillard reaction to occur in the malt, similar to what happens to bread crust during baking, but they pull it out before it gets burnt. By contrast, a porter malt will be raised quickly to 450°F and kept for 2 hours at that temperature, until it is charcoal black.
Part of the reason that craft German brewers don't completely toast the Munich malt is so that the malt retains some diastatic power to break down the starches in the grains into simple sugars for fermentation. In other words, the lighter the roast, the more enzymes are preserved for the fermentation process.
If you were to completely toast your malt, you would have to use another, lightly roasted "base malt" to allow fermentation to happen, essentially using the toasted malt for flavor only.
A dunkel brewer finds the right balance in the roasted malt, so that he only uses one malt for his brew. Why does he care? For one, the lower diastatic power of Munich malts means a superior, full bodied mouthfeel in the final product. He does not want over-attenuation that would make the beer too watery. On the other hand, this traditional way of making a one-malt dunkel goes way back in time, and that is a good enough reason for some.
Besides the malt differences, there are also different yeasts. English porters use top-fermenting yeast to create an ale, while the German dunkel uses bottom-fermenting yeast to make lager. The slower, gentle aging process makes the dunkel a completely different animal than its English counterpart.
Ale fermentation brings out bolder, stronger flavors because the fermentation happens at a higher temperature and more quickly. Often ales have an overwhelming characteristic. The dunkel has more subtle flavors, but with more complexity. In a dunkel you will notice a myriad of subtle caramel and malty notes, besides hints of coffee and chocolate. Depending on what you are looking for, you prefer an ale or a lager.
Germans for the most part believe beer should be easy to drink. They prefer lagers because they want to drink a few beers at time, without being fatigued by strong ales. As Franz Josef Göller, the retired father of the current brewers, was fond of saying, "You know a beer is good if you still feel like having a second one."
To bring this to and end, the dunkel bears little resemblance to a porter, other than its color. If you tasted them side by side, you would immediately notice the huge difference. Of course, each person has his or her preference, but I absolutely love the perfect Dunkel from Göller.