Neither of those sites have a 2010 2.8 GHZ 4 core available for comparison.
Barefeats does indeed.
Right now, for most users, clock speed still trumps the number of cores you have as most programs hardly take advantage of more than one.
Per Mac Performance: "A 3.33GHz model runs 13.6% faster than 2.93GHz, and 38.7% faster than 2.4GHz. A 13.6% clock speed difference means about 68 seconds vs 60 seconds, or 1.13 seconds vs 1 second — noticeable, but not decisive. But 3.33GHz vs 2.4GHz is substantial eg 83 seconds vs 60 seconds, or about 1.4 seconds vs 1 second — now that you'll notice."
I'd wait and see regarding FCPdon't try and factor its performance on the machines right now. As I understand it, the app is due for an update and is currently not doing much to use the horsepower available to it. But if you look at apps like Logic and After Effects, you'll see the difference optimization makes. Logic on the 8-core outperforms the 6 and AE is neck and neck. Photozoom scores on MPG are also another example of the difference optimization makes.
It would be reasonable to assume that next version of FCP will be way faster on the 6-core than the 2.8 quad. The 6 is faster now and will be even faster as time goes on. If you can afford it, it's a better machine.