Probably neither. If someone tells you otherwise, ask them to back up their claim.
Nowhere in the official documentation does Adobe mention CUDA or OpenCL support. Not in the Illustrator-specific article on GPU performance, neither in the system requirements.
Adobe only mentions that Illustrator CC 2017 and its predecessors use OpenGL on both macOS and Windows:
While we’re at it, starting from CC 2015, Illustrator’s GPU Performance feature isn’t limited to Nvidia videocards, i.e. AMD and Intel GPUs are also supported:
For the 2015 release of Illustrator CC, GPU Performance is available for both Mac OS and Windows, and it works with various GPUs. (Source).