Not $2,000 faster
Hard to say. By my math, with a D700 and a 1TB SSD in the system..
The 8 core is 37.5% more cost, than the 6, but also 33% more processing power.
The 4-6-8 cores are all pretty impressively even performance wise, it just really comes down to will you use the 7th & 8th core or 13,14,15,16th thread?
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I keep going back and forth between the 6 core and the 8 core. And do I do enough rendering and encoding to justify the $1500 upgrade? Nuke, Modo, LW, AE, FCP, Motion, Handbrake will all benefit from 8, Mari, Zbrush, Marvelous, Substance, PS will all do fine with 6.
Its 37% more cost, but 33% more processing, plus seems to have the highest single thread score - probably from the huge L3 cache which is twice the size of the 6's -which I think is why apple sent out the 8-cores for people to review.
Is the 8-core going to have a much longer lifespan then the 6?
If I get 4 years out of the 8-core its an extra $31 a month for ownership.(which I think seals the deal in my mind to go 8)
In either case I'll be putting in D700's and the 1TB SSD.
I'm still on a MP 1,1 right now, so I'm not sure if I'm over compensating for painful render times the last few years. So in either case I'd be upgrading to a system between 3.5x an 4.7x faster than what I have now just in CPU.