It all still comes down to the i5 being mainstream and the i7 being more of an enthusiast chip. I'm sure you can OC the i5 to perform better than a stock i7, but you won't top out an OCed i7 at all with an i5. To bad they are using 2 different chipsets though. But that's fine.
I'd say if you are in the market for a higher end system, the i7 is the way to go. If you are looking for mainstream the i5 will be the replacement for the current C2D machines.