I have both the 2012 and 2014 models. I haven't noticed a big difference between the two in terms of graphics performance but don't spend a lot of time on the 2014 (it's mainly a DVR/transcoding computer) and much of the time I screen-share when I have work to do on it. What (if anything) are you doing that you notice any issues with the 2012? I have two different versions of Adobe Premiere Elements (video editing software) - one on my 2012 Mini and another on a computer with a much faster GPU. I mainly edit 720p videos and for what I do I haven't noticed much of a difference in graphics performance between the two systems. It's not that there isn't a big difference in GPU performance, it's that the nature of the work that I do doesn't overly tax the 2012 GPU performance that I notice the difference - so it might not be worth it just for the extra graphics performance of the 2014 Mini. And for video encoding that uses the CPU (rendering a video to a file), if you have the quad 2012 Mini, your performance will suffer in going to the 2014 Mini.
I think you'd need to investigate the specifics of how After Effects and the Mocha (plug-in?) works. If you're using the plug-in bundled with AE, that's "free" whereas the regular version is $595. Anytime I see that, I wonder what's missing from the free version - there may be a performance component to that.
This is just one article on the web I found - the last poster says the GPU makes no difference - I don't know if that's true but you can get an idea as to how the GPU is used in AE by reading this and other similar articles or perhaps post to the Adobe site to get recommendations. https://forums.adobe.com/thread/1916149
I have been using a 2012 2.6ghz quad core i7 mini with 16gb RAM and 1tb SSD with the mocha based TrackX and SliceX Final Cut Pro X plugins. Seems to work pretty well, but faster would be better. I was under the impression that the graphics chip wasn't as significant as the CPU. And this model has the fastest CPU of any Mini - about 1.5x faster than the top of the line 2014 3.0 ghz i7 Mini.
Don't have any experience with After Effects unfortunately. But for any demanding video work, there are surely some better choices than any of the Mini's.
Any halfway serious GPU-related stuff on any mini would probably need an external GPU in 2018, until Apple unwraps an updated mini this or next Year.
At least nowadays it seems halfway affordable (Akitio Thunder and an older-gen GPU) and doable even for an average user to go eGPU. Whether it would make economic sense to spend that time and money for a machine that’s technically a couple of years old, is a different question, though