8-core is better in the long run. Snow Leopard is optimized for multiple cores and will use them more efficiently.
More cores is definitely better when running multiple apps. Your "workflow" is the combination of all of the apps you move between during an average session, whether or not they are related to the same project.
Although you may spend 45min focus in After affects, once you set up a render, you may jump between emails, open multiple web pages, preview quicktime movies, open other audio or video apps, and possibly have another app rendering at the same time. That render is using 40% of your total CPUs, leaving 60% for the rest of your work. You can actually get something done while rendering, thus you as a user are now more productive, even if the 8 cores are not rending twice as fast as the quad core.
Though I agree on the necessity for more cores concept for video/graphics workstations, SL's not going to make much of a difference (Grand Central aspect anyway).
Most applications that can benefit from multi threaded operation, have already been written to accomplish it. So the API calls added to SL won't make a difference. Then there's the fact those API's are more general purpose, and not optimised to a specific task, which can be done by the application developer. Where it can help, is aid the developers who've not yet done it for apps that can. Stragglers and fence sitters per se.
It's not going to be able to take single threaded apps and make them mutli threaded apps either, as most are for a reason. Sequential operation, so the next output is derived from the previous data. It can't be split up due to data dependencies.
Where SL will help, is the fact it's a 64 bit Kernel, and is optimized, so the 32bit code bloat is reduced (perhaps not able to be entirely eliminated to retain some backwards compatibility). That would be quite welcome IMO.
Then there's the future possibility of Open CL aiding renders for example. But that will take time to come to fuition, as the code still has to be developed by the application developer to use it.
All in all, SL will be a welcome improvement, but not going to provide substantial performance gains on it's own. Software has to be rewritten/patched to make use of it, provided it can even help.
If you're wanting to speed up renders, you'd want to look into improving I/O throughputs, such as increased memory, and disk (i.e. RAID), and faster clocks (processor swap, assuming it's feasible financially).
