[nl]

I've used both, although not Solr 4.

Elasticsearch has easier config, especially for clustering. It is designed to be schemaless so you can push almost any JSON data into it.

Performance is adequate in both.

It's not a big advantage either way, unless you need clustering.

[ejain]

Elasticsearch isn't really schemaless, but you can have it guess and extend the schema as fields are encountered (or you can have it treat specific field as blobs of unindexed json).