The instance types you refer to are contradictory so I'm not really sure if this is really about Arm-based servers. The i3en-vs-r6 is not an apples-for-apples comparison.
The R6g type is EBS-only so they will perform significantly worse than i3 instances. R6gd come with NVMe SSDs but they are disproportionately small compared to the CPU+RAM they have. For example, a r6gd.2xlarge which has 8 cores + 64GB RAM only has a 474GB NVMe SSD so they're not a good back for the buck. On the other hand, i3en instances are intended for dense storage. I'd discourage you from choosing this type since it will be tempting to have dense nodes and are problematic when it comes to operations such as bootstrapping, decommissions and running repairs. For example, an i3en.2xlarge with 8 cores + 64GB RAM can potentially have 5TB of disks (2 x 2.5TB NVMe SSDs). In my experience, i3 instances are the optimal choice such as i3.2xlarge. I think 8 cores + 61GB RAM + 1.9TB NVMe SSD is the sweet spot for price and performance. Cheers!
