I'd recommend going with the i3en instances actually. Splunk Cloud is in the process of moving to these, as compute and storage is a bit denser.
Regarding if you want to use raid on these or not, this is pretty much up to you and depends on the instance types you use. If you use a i3 4x vs 8x your getting 2 vs 4 1.9tb disks. If you drop those in RAID, you loose space. If you go raid, which we do in Splunk Cloud, you need to size your cache appropriately. Which usually means adding more indexers.
That being said, if you have autoscaling or automation setup properly, your indexers could loose an instance and rebuild with someone minimal impact to the cluster. Since all data, aside from hot, is in S3, adding new indexers and rebalancing is pretty minimal system impact..