The Cantons are a better deal for sure. It wasn't like I went looking for the Sierras so much as they fell in my lap. That said I do have some SVS Ultra speakers for sale...
You know, once you get up to this range of price and quality, the differences between speakers become smaller and smaller. The Sierras weren't this big huge leap over the SVS Ultras, but I could hear the difference and to me it was worth it. Diminishing returns are a pretty subjective thing. I think if I were where you are right now I would probably go for the cantons for that free Center Channel. That's going to be a really nice upgrade for you I think. By all accounts they are at least on par with the Ultras, if not better. I was very happy with my Ultras until I heard the RAAL ribbons.