I’m the product manager for both the Couchbase ES connector and for FTS.
The most important difference is that FTS is currently DP and it will only go GA once it meets our internal performance KPIs. We don’t expect FTS to hit those KPIs in time for Couchbase Server 4.5, so when Couchbase Server is GA, FTS will be shipped with it but will remain as a DP feature.
To your question:
Both systems can handle the use cases you mentioned and handle them in similar ways. When it comes to scoring, FTS uses TF/IDF scoring and supports query time boosting. FTS scoring will be very familiar to most people who have used Solr/ES/Lucene. ES has more possibilities to customize its scoring, although that’s unlikely to be super significant in case you’re mostly using prefix and fuzzy search.
ES is a powerful and mature search product, so there are things ES can do that FTS doesn’t do, and more things that you can customize with ES. A full comparison is quite long because even though FTS is a new component for us, it’s been in the works for some time and has a lot of functionality in it too. Some obvious things ES has that Couchbase does not: support for the ELK stack (Elasticsearch / Logstache / Kibana), percolation, a query DSL, manual control of sharding are a few that spring to mind.
With FTS we’ve tried to stick to straightforward and common search use cases and make those easy. Easy in this context means not requiring a customer to set up and manage another system, manage transport between systems, secure those systems, and distribute and replicate indexes between systems (note that distributed indexes were not shipped in the Cb 4.5 DP that you can get today). Ease of management is the main advantage that FTS might have over other systems when you just look at the search features.
Hope that helps,