What are the major differences between solr modules I am looking into Solr search for my site and am seeing two equally well-used modules.


Apache Solr Search (lots of contrib around this)
Search API Solr Search, built on Search API


Why would I pick one over the other?
I would recommend using Search Api Solr. Search Api Solr is a little bit more complex; however, it is more powerful.  In addition there are many sub-modules that allow you to extended the search funcionality.

You should consider reading the article Battleplan for Search &amp; Solr in Drupal 8.

According to this post. 


  The two maintainers of Search API and Apache Solr have meet in person and have determined a way forward for advanced searches with Drupal and they both have agreed that Search API is it.