Id love to help, I was in the exact same situation and no one helped me. I also have the true bass and used it for both. What headphones do you currently have?

Firstly, build wise I trusted myself with the 660s more, no doubt. While the sudara was metal, the yokes felt thin and I felt that if I dropped the headphone too many times or was a bit harsh I could break them. 660s did not feel like that.

Comfort, another win for the 660s. The Sundara were not bad, I have a small head so they were a little loose but they would be fine for most people, but yeah again the 660s were just way better for comfort. Not as heavy either if that matters.

The cable on both are fine, the Sundara is not a material I personally like but there isn't anything wrong with the cable (does not kink). The 660s cable I much prefer (as a 58x and 6xx user) but it is way too damn long and I hate that. Take what you will of that.

QC could be there for either headphone, but id trust 660s to last longer. I don't know what country you live in but for me it's not easily replaceable so senny for me.

Looks are something no one should care about but yeah for me 660s look much nicer and much more sleek. Sundara looks wack.

The objective stuff, the soundstage on Sundara was much wider, and it was fun to listen to. People say the 660s are cramped in comparison, but to be honest I loved both (660s are wider than 6xx). Both headphones represent the soundstage in their own way that is nice to hear. I personally like intimacy more so 660s was the winner but Sundara is quite good too. Take your preference

The imaging goes hands down to the 660s, it's way too good. The Sundara isn't bad but 660s really outshine it. So for competitive gaming it would be better (I personally judge that imaging is more important than soundstage for this case).

SOUND
Ill try my best, but the 660s to me felt cold: I think all the frequencies were present (though it sometimes felt something was missing to me), but there is a good amount of detail. The midrange is the pure focus and it does it excellently, with the bass being alright in both quantity and quality. Treble is good for those who are sensitive to treble, as there isn't that much and it's a relatively smooth curve. It's a good all rounder headphone.

The Sundara does the highs much better, they are smoother and more detailed, but it can get hot for some people (like me). The mids are not as good as the 660s, definitely less focus, good detail but not as smooth and so it is not as relaxed, though this isn't a bad thing for some people. The bass has less quantity but better quality.

Which do I prefer? the 660s because of the highs, I am sensitive to sharper treble. But which would I recommend for people for pure sound quality? Sundara just because of how much detail is present in the headphone. There is SO MUCH at that price point and it really does outshine the 660s in that regard. Though if you prefer something more luscious in the midrange the 660s wins there. If you are more of a bashed, neither of these would be my pick and I would go with the hd6xx and the true bass, but if you had to go with one of these then 660s.

This is all written really quickly and I haven't reworded or reread anything, so if you have any questions please do let me know, I do not mind answering.