I can't speak to the Sundara at all, but I find the DT 1990 to be a really great headphone.
The treble is definitely emphasized, but I've never heard it to be harsh or sibilant. I have a few albums that it's bright, but never overbearing. At least to my ears. In fact, over the 6+ months that I've had the DT 1990, I think it's become more natural sounding. Maybe it's me, maybe it's burn in.
Bass reaches lllooowww and still hits hard and remains incredibly detailed. A little while ago I was listening to some hip hop and it was enough that I was actually feeling disoriented! All the while never feeling bloated or misplaced.
Mids are great. Very detailed and linear, if very, very slightly recessed. To be honest, they only feel recessed if I directly compare them to the HD 650, which are pretty forward with the mids.
Treble. Yeah, it's forward, but to me it sounds very correct. The only time I find it remotely offensive is on a couple electronic albums and some poorly recorded acoustic with inappropriate upper harmonics, and even then it never borders on sibilance. Mostly, coupled with the dynamics, the treble allows these headphones to play aggressively when needed. Distorted guitar or rock vocals are portrayed with realistic grit. Personally, I'd imagine the only way you'd find the treble offensive is if you have a particular sensitivity to it or you really just want a more relaxed presentation.
Dynamics are phenomenal.
Soundstage is intimate, though imaging seems precise.
Detail retrieval is astonishing, though you could also say it's unforgiving. These are professional headphones, after all.
These have a very neutral presentation that doesn't add much to the music (other than treble) to make it more appealing.
Comfort is decent. I can usually listen for a few hours before I feel like I need to take them off.
Build quality is top notch, with all parts being replaceable. I believe that HiFiMan are supposed to be better built than they used to be, but all the horror stories of poor QC would keep me away. I think there were a few people with the DT 1990 that complained of creaking earcups/yokes/whatever, but I don't recall reading about any other build problems. On mine, the left earcup is slightly tight, but that could be easily fixed by loosening a screw.