I personally would go with one really good subwoofer over 2 lesser ones that cost the same total amount, in most cases. First, the great advantage of multiple subwoofers is allowing one to even out the response in a room, but if one buys subwoofers that do not give a flat response in the first place, then one has more problems than just room effects. And this brings me to another reason: Most people do not have the space to put subwoofers just anywhere, and so they end up putting them where they have to go. Unless one is incredibly lucky, this means that one will be placing the multiple subwoofers in the wrong places, which greatly defeats the purpose of having multiple subwoofers. A third reason to favor one really good subwoofer is the fact that most people upgrade over time, and if you have a bunch of lesser subwoofers, the upgrade is expensive and troublesome, as it involves replacing all of the multiple subwoofers with better ones. But with one really good subwoofer, the upgrade is simple and easy and cost-effective: One simply buys a second subwoofer just like the really good one that one already has.
In the case of BIC subwoofers, I would pick one SVS over a pair of them any day. Even if I could never upgrade.