Both are amazing headphones, but the safest option would be the HD600. There's a reason they've been a staple in this community for so long. If you're not very sensitive to treble though I'd take the DT1990 because it is a bit more exciting to listen to (better for genres like EDM or dubstep). Soundstage and imaging is also substantially better on the DT1990

Also, I wouldn't really say you will hear that treble spike even if you do have sensitive hearing. I have excellent hearing and my DT990 (which people say is even worse treble-wise than the DT1990 Pro) sounds perfectly fine to me. Maybe I might experience slight sibilance on 1% of songs, but overall I hear no harshness