Given the storage formats are identical, you should choose the most cost-effective option for your workload now and scale that to larger machines (ARM or x86_64) when it’s justified.

I promise you, you’ll know with plenty of time to spare before you’re going to hit the current 64-core limit of Graviton before you need to decide if you can fit into a 128 vCPU (which I believe are hyper threads not cores) x86_64 instance.