It seems Graviton 4 CPUs have 12-channels of DDR5-5600 i.e 540GB/s main memory bandwidth for the CPU to use. M3 Ultra has 64-channels of LPDDR5-6400 i.e. ~800GB/s of memory bandwidth for the CPU or the GPU to use. So the M3 Ultra has way fewer (CPU) cores, but way more memory bandwidth. Depends what you're doing.