TL;DR definitely GPU
“I intend to use it mainly for rendering, rigging/animation and sculpting. What should I go for?”
There are many good answers to this question if you look up a Quora question “What-roles-do-the-GPU-and-CPU-play-in-3D-rendering”
The GPU is what actually creates the output to your monitor, what I am certain of is that you want to do sculpting, and real-time editing of animations, and you want a fast response time, the better GPU is preferable to more RAM. If you had a lot more RAM, you might have an advantage in the maximum amount of objects you can have in a scene without any issues.
Multicore CPU is the number one priority, by the way. Your question is on GPU memory or regular RAM, but it’s still worth clarifying. Multicore processors are generally more useful for CAD/3D modeling than they are in videogames, because the mathematics of CAD is more “predictable”, so it can be divvied up more effectively across multiple cores.
I’m also fairly certain that the GPU would be better (faster) for rendering as well, but there are slight differences in visual appearance for CPU vs. GPU rendering, so it’s subjective.
CPU: does “math”, literally, it computes things for your computer.
GPU: outputs “math” to your monitor (which requires its own kind of computation)
RAM: contains information that your CPU needs but not immediately. Your CPU stores the most immediately important information on its cache, which can be thought of as super fast RAM, but it’s more expensive, and it can’t store as much data.
If you were doing a bunch of statistical data analysis, CPU & RAM alone would suffice. Oh, and just to clarify, RAM never makes anything faster, it just prevents a resource shortage that makes things slower (or unworkable). Although that’s a semantic point, so I’m sure someone could dispute it if they were so inclined.