It looks like mongodb offers two similar functions for geospatial queries - $near and $geoNear. According to the mongo docs
The geoNear command provides an alternative to the $near operator. In addition to the functionality of $near, geoNear returns additional diagnostic information.
It looks like geoNear provides a superset of the near functionality. For example, near seems to only return the closest 100 documents, whereas geoNear lets you specify a maximum. Is there a reason to use near instead of geoNear? Is one more efficient than the other?
Answer
Efficiency should be identical for either.
geoNear's major limitation is that as a command it can return a result set up to the maximum document size as all of the matched documents are returned in a single result document. It also requires that a distance field be added to each result document which may or may not be an issue depending on your usage.
$near is a query operator so the results can be larger than a single document (they are still returned in a single response but not a single document). You can also set the maximum number of documents via the query's limit().
I tend to recommend that users stick with the $near unless they need the diagnostics (e.g., distance, or location matched) from the geonear command.