It really depends. Currently I myself am looking to upgrade an almost 13 year old machine (i7 920), and the only reason why I did not have to do this sooner, is because I rely on a relatively new GTX1080 to render in GPU render engines like Cycles, ProRender, Octane, and LuxCore.
As a C4D user your native GPU renderer is ProRender, but C4D's version is not that mature. The internal classic render engine is purely CPU-based, and aging.
If you have access to Redshift, the choice would be simple: go for GPU rendering. If you decide to stick with the old classic renderer, a fast CPU is your only choice.
A good GPU may however find good use in overall daily use: it can vastly reduce video conversion times (Handbrake + CUDA), and is essential for a fast viewport in C4D. NEVER GO WITH ONBOARD and 3d apps if you can help it.
And Nividia's new RTX3070 GPU is only going to cost $499, which is an absolute steal (it is faster than a 2080ti!). Then again, the cost of current 2070 and 2080 cards are nose-diving, so perhaps a second-hand card may give you both a fast CPU AND a fast RTX20XX card.
I can only speak for myself: I will upgrade to a VERY fast 3090, and save money by picking a slower Ryzen CPU like a 3950. (Yes, still much more expensive than what you would have in mind I guess, but I hope you understand my reasoning: I could go with a 3970 which was my original plan, but that new RTX3090 card is really a steal compared, and will have applications for me beyond GPU rendering).