I go back and forth between them every few days, and every time I switch, I go "yup this is my favorite." Probably the 660S by a hair though. If I could only have one it'd be it because it's more of a catch-all for my music preferences.

The DT 1990 is more aggressive and punchy (in a good way), the staging is more open and wider by bit (660s is decently wide but I perceive it more around my head than infront of me, both are cool with me), it's also clinically analytical where the 660 is warm and dark. I feel like the 660 gives music it's own presentation, while the 1990 just shows you it as is.