Despite the Geocoder having a bigger community and giving the impression that it's better in all aspects, the majority of our developers have had less trouble using geokit-rails.
The Geocoder queries use aliases for computing geo-information (e.g., distance). When used with the PostgreSQL database, this implementation detail causes bugs in the client code in cases when results need to be sorted by the computed geo-information (because PostgreSQL doesn't allow ordering by aliased columns). We have decided to use Geokit-rails until the maintainers show initiative to address this problem.
For more detailed information about the state of the gems, please have a look at these gem comparisons