8 cores is nice but unnecessary. Adobe products rarely utilize all the cores unless you have multiprocessing enabling, which if you don't know: multiple frames are being rendered at the same time using different cores for each frame. But even at that I doubt you could get all 8 cores on board to do that because you need more ram dedicated to each cores processes.
An addition of .5Ghz for a sacrifice of 2 cores sounds like a deal to me! Plus if you choose to multiprocess you can utilize almost all of your cores, given you get the 16gb upgrade.
Now the video card is simply an upgrade according to benchmarks. With the +.5ghz on the CPU and an upgrade to the D700 it basically makes it render at an equivalent speed of a 8core 3.0ghz & D500
I know you didn't ask this but I think it changes everything: get an SSD (Solid State Drive). I have one in my PC and the boot up time is 5 seconds, I can only imagine the boot time with a light weight operating system like OSX. Applications open just as fast and finding files in your project/loading your project in a second. Not going to ramble on because I don't actually know too much about SSD's other than first hand experience. I do however recommend you go to this article, it has great visuals and explanations on SSD vs HDD!
In conclusion: I would get the 3.5ghz w/ Dual D700 and if possible, a solid state drive (ssd).