The trilogy is more colourful and cinematically appealing. The Dekalog films were never meant for film release, Roger Ebert says they were never seen in the cinema in America, and look quite drab, which is quite appropriate given the dismal, winter public housing setting.
I like the stories. One problem may be that it's difficult to watch all ten parts in one go, and breaking them into several viewing sessions reduces the overall impact. I've seen the trilogy so many times, I thought I'd have another look at the Dekalog instead.