I am listening to the Denon D2000 as I type this. I just compared it to the DT990 ten minutes ago. The DT990 is much, much brighter, has more powerful bass and a larger soundstage. The D2000 has a much smoother, warmer sound with clearer mids. The DT990 needs an amp. The Denon is much better with an amp but is not grating without one. The DT990 is open back and the D2000 is closed. They are both comfortable. The DT990 is lighter and the Denon has bigger ear cups which fit me better.
These two headphones are different enough that you are almost choosing between apples and oranges. Neither one would be a wrong choice for me. Of course, both of them is a better choice. This is a long way of saying you would not go wrong with either but I think it would be very difficult to really know which you would ultimately prefer without being able to hear both.