Are the Sierra towers that much better to justify paying 3 times as much?
That is a very difficult question! First off, I think we know that the Seirra's won't be 3 times better (as if we can actually quantify "better" in any precise way). But lets assume we can actually quantify better with precision...
How much better would the speakers need to be to justify the added cost? If they were 15% better, would you pay 3 times as much? Lots of people would and lots of people wouldn't!
The thing is, only you can decide what a dollar is worth to you and what a SQ improvement is worth. And that is really the right way to decide - get the speakers in your house and listen. Ascend has a free trial period, but you end up paying return shipping which could add up for floor-standing speakers.
I don't know how flexible you are, but SVS will pay return shipping if you don't like their speakers, so it is a truly free opportunity to do a comparison! I have heard their Ultra Bookshelf and it is quite a nice speaker for the money. I would expect their tower to follow suite. These have had very favorable reviews, but have not been out so long as the Sierra's.
TheWarrior did a write-up that covered the Ultra Bookshelf here:
http://forums.audioholics.com/forums/threads/bookshelf-speaker-shootout.95335/