The DT 990 is open back, the DT 770 is closed back. The DT 880 would be inbetween those (semi open).
What is better depends on your use case. The simple difference is:
Open back has a bigger 'sound stage', so in gaming or a movie you can much better hear where a person is walking.
Closed back has less of a soundstage, meaning it is less fit for gaming for example (although this is of course based on personal preference).
You have to keep in mind though, that open back headphones leak sound, like.. leak a lot. I have a DT 990 250 Ohm (which I love by the way), but I couldn't use them in public, because you can basically hear everything I am listening to.
Otherwise the DT 990 is a super comfortable headphone, just gotta mention though it has a (old) telephone style cable on the 250 Ohm version, which are very compact, but could feel heavy and run awkward on a desk.