You are processing huge amounts of logs continuously. You care more about throughput than random IOPS. Which volume?
For processing huge amounts of logs continuously where throughputs is valued over random iops, the best volume type is Throughput Optimized HDD (st1).
Log processing is a classic sequential write and read workload. Because logs are appended line-by-line in a continuous stream, they rely heavily on high throughput (large data blocks moved per second) rather than fast random seek times (iops).