My bet is that performance-wise, they'll basically be even at identical clock rates.
HT gives about a 40% boost on WCG, along with the i7's slightly higher IPC, it should level out.
It remains to be seen how far the X6 can be OCed, and how power-efficient it is - price/performance is definitely in favor of the X6, considering the motherboards are about half the price of an x58.

However, as soon as affordable Intel Hexas are available, the game changes again. They do 50% more than a Bloomfield, clock very high, and draw even less power.
Right now, 32nm Westmere is unbeatable when it comes to performance per Watt (Dual X5650: 72000 WCG PPD @ 300W from the wall)