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.

Thaxll is not wrong and shouldn't be downvoted. Vertical scaling means more cores, not higher single-thread performance. Because Graviton is limited to 1-socket systems, EC2 has x86 instances that are larger than the largest Graviton. Whether you'll ever need those is workload-dependent.

Well, no, he is wrong. Postgres can run on either x86 or ARM. You can do replication between them, no problem. There's no reason to run workloads on x86 just to get access to larger instance sizes at some point down the road. You can migrate Postgres on ARM to Postgres on x86, it's not even hard. Use what works best for you now and in the part of the future you have some visibility on. AWS might well announce bigger Graviton instances before you ever get there, if you ever get there at all.

single instance != single core/thread