A couple of reasons, one is kind of fickle. The first is that the Rockets sound on the warm side of neutral which I tend to enjoy for movies and jazz. I could listen to them for hours without fatigue. I always thought voices sounded more realistic with the Rockets. The Rockets are definitely the stronger performer with movies but that's to be expected with an extra driver. The second reason I prefer the Rockets is that I enjoy the way they look a little tiny tiny bit better. Mostly because I dont enjoy the look of stands. Both of those are personal preference, but pictures really don't do the veneer justice on the Rockets.
For music I prefer the Sierra's they really seem to bring performances to life and touch your heart. My only complaint is that occasionally, very occasionally, the Sierras will sound a tad bright. But honestly Nora Jones has never sounded better. The finish on the Sierras is also stunning. My girlfriend laughs because they have never been touched by human hand, you have to wear the gloves to touch them. O I would also give Ascend the advantage in the stable company department, I don't worry if they will be around tomorrow and their customer service is excellent.
In comparing the two. Like I said I truly do think they are of equal value, both are fantastic. Asking me to choose one would be like asking me to choose a favorite child. Both have great highs and an unexpected low end. Both are heavy and seem to have excellent build quality. The looks from both are stunning. I think of a speaker as a piece of furniture so for me they need to look good and they do. I should say I have had a blind testing for my friends and the ones that could tell a difference were split pretty evenly.
Another suggestion would be the Era Acoustics Design 4. I have never personally heard them but I have heard excellent things about them and they seem to fit more in the lower price range.