I don’t think bucket rendering has anything to do with memory–when I render with a bucket renderer it still uses the full amount of memory required. I don’t know of any GPU renderer that uses buckets or scanline, but Redshift has some way of using system memory if the GPU runs out of memory, I’m guessing that lowers performance though. That’s the only GPU renderer I know that doesn’t rely solely on the GPU memory.
Also–CPU cores and GPU cores are not the same thing, having more GPU cores doesn’t automatically equal more performance than less CPU cores.