How is your current system performing? Look at your cpu, memory, and disk utilization for any constraints. Search heads tend to be cpu/memory bound and indexers tend to be i/o bound. If your current system is not running out of resources, you probably don't need to expand. According to the Performance Recommendations doc:
An indexer that meets the reference hardware requirements can ingest up to 300GB/day while supporting a search load. For a review of the current reference hardware specifications, see "Reference hardware" in this manual.
There is added complexity in managing a multi-server environment. It's not difficult, but if you don't need to switch, stick with a single server. Yes there are probably situations where running a single search head and single indexer will provide some performance advantages, but really, taking that step should be because you plan on expanding past the 300 GB/day mark and will need to add multiple indexers.
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