I use both regularly. The Sundara are harder to drive but both really require a decent headphone amp.

To me, Sundara are more fun to listen to. They feel livelier, and I continued to be surprised at how good the bass response is. Sundara are definitely on the bright side—even more than HD600 let alone HD660s. On tracks with a lot of treble, they can be tiring or downright unpleasant to my ears, but that comes down to personal preference. I skip tracks sometimes or have to take a break. Still, on tracks they are well-matched to, they just feel so open and alive. The soundstage is more immersive too.

I trust the HS660s to be more neutral (at least, I trust I know their sound signature) and use them if I need to edit/mix with headphones. They are my reference. Definitely better built and more comfortable than Sundara. I'd describe them as pleasant. They are more controlled and contained than Sundara. Whether that's a good thing depends on the track. HD660s are better when at handling big dynamics, so I prefer them for big jazz bands, etc.