All I can tell you for sure is that they are working on it and it will be much improved. Will the next release be perfect? Probably not. Will they reinstate the old render multiple frames simultaneously? Definitely not. Will it take all the plug-in manufacturers a while to catch up? The answer to that is yes but some like Trapcide with Tao and Particular are already way ahead in some areas. I think it's too early to worry about precisely matching a CPU to a version of the software that is not released yet.
What software do you use other than After Effects? This makes a big difference. Especially since Cinema 4D comes with After Effects now, you need to consider that. Cinema 4D can use all of the cores you've got. So dual Xeons might be the better way to go if you plan to do lots of work in C4D.
What kind of work do you do? If it's mostly stuff with Element 3D, it renders on the GPU, so the i7's might be a better investment and you can put more money towards powerful GPUs.
Even if neither of those things apply to you, the recommended workflow (until the AE team gets a new multi-core rendering solution in place), is to work in CC 2015 (faster interactivity) and then render in CC 2014 (which does have multiprocessing). So the dual Xeons might be better.
However, if you don't use Element or C4D very much, perhaps snappier interactivity and previews are what you want, so (if the i7 has a faster clock speed), it may be the better option.
To answer your actual question, none of the NAB presentations or sneak peek videos have indicated that there is a new renderer in place that uses multiple cores.
So, if AE is your only concern and you want to focus on faster preview speeds in CC 2015 (and whatever the next release is called), go for the highest clock speed you can get. If, however, you plan to do 3d work, you should probably get the dual Xeons (and CC 2014 will render faster on it too). That is, of course, if the clock speed multiplied by the number of cores is significantly faster with the dual Xeons. I mean, if it's dual 6-core Xeons running at 2.0 GHz vs. a six-core i7 running at 4 GHz, get the i7 all day long! Many things in C4D, AE, Premiere, and pretty much any other software are still single-threaded.
Thanks Rick. I was hoping with the sneak peek it would no longer be too early.
Thanks Szalam. As I mentioned to Rick, we were hoping that the sneak peek would have provided more information on addressing multiple cores. Our C4D work goes through Octane which uses the GPUs for rendering. Still - point taken about increasing interactivity performance while working in the program.