Elasticsearch is the more common of the two in our index: it runs on 181 websites against 4 for Segmentify. Counts come from real scans, not vendor claims.
Elasticsearch is a search engine based on the Lucene library. It provides a distributed, multitenant-capable full-text search engine with an HTTP web interface and schema-free JSON documents.
Segmentify is an ecommerce personalisation platform.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.