I have found that Sun rims are softer than Mavics. If you are a big guy or just a hard rider I doubt the Single Tracks will hold up to you. As is my rear Single Track has a fairly good sized ding and a flatspot after about 5 rides. I have had a 729 on the rear of my Bullit for 2 or so years now and after many times downhilling and aggressive riding it is still true and dent free.
Those black 729's do look nice. I remember seeing pictures of them on VP Frees a couple of years ago but never actually saw a black 729 in person.
Now, about 521's/721's. I have a pair that have almost two years of hard riding by a big Clyde, that's me. They have not gotten one flat spot and have never gone out of true. I believe a lot has to do with the quality of the wheel build but I have never had rims hold up as well. My D321's(Same as current 729's) have not heald up nearly as well. They are still awesome rims but they do have dents, flatspots and my rear has a nice little hop to it. So, I'm pretty stoked on D521's/721's.