http://www.bostongis.com/PrinterFriendly.aspx?content_name =sqlserver2008_postgis_mysql_compare
From the PostGIS Listserv comes a very useful comparison table that has been partially vetted by the members of the list. I just wish they included Oracle Spatial and Oracle Locator.
Paragon wrote:
 /> As part of our due diligence work, we are formulating a comparison of the
 /> spatial functionality offered by SQL Server 2008 Spatial, PostGIS and MySQL.
 /> Its currently in draft, but we hope to release within the next week.
 />
 /> We would appreciate if people could take a quick look at it and let us know
 /> if you see any gross errors or important things we left out. Most of this
 /> information we curled from reading the help docs of each product and from
 /> our general familiarity with the databases and GIS products out there. We
 /> tried to be as fair as possible.
 />
 />
 /> http://www.bostongis.com/?content_name=sqlserver2008r2_oracle11gr2_postgis15_compare#221
 /> tgis_mysql_compare
 />
 /> Thanks,
 /> Regina and Leo