What is the difference between searchkick and elasticsearch-rails?
Searchkick is much more Rails friendly gem, they have features to get autocomplete for example and don't force you to learn guts of ElasticSearch to do basic stuff.
Also, looks like guy who wrote searchkick use it himself in Rails vs Elasticsearch-rails is just library for one more language for ElasticSearch team.
I started with Elasticsearch-rails and migrated to searchkick myself.
elasticsearch-railsis maintained by group of people working in Elasticsearch.org.
All libraries for Elasticsearch for different languages are using similar patern, so if you switch for example to python, structure of it's library will be simmilar.