Scaling-wise? Distributed Elasticsearch doesn't have a Zookeeper dependency, which is nice. But Solr has more sharding flexibility and partition tolerance.
Like everything else, depends on your needs.
Yes, see [1] and [2] for a comparison between Solr and Elasticsearch. I should mention also that ES are working on the partition tolerance issue as described in [3]. I am currently using 1.4.3 and am wondering if anything was addressed in 1.4 for resiliency.
[1] http://lucidworks.com/blog/call-maybe-solrcloud-jepsen-flaky... [2] https://aphyr.com/posts/317-call-me-maybe-elasticsearch [3] http://www.elasticsearch.org/blog/resiliency-elasticsearch/