By request, here's a repost of the summary of geographic software for Plone that I sent to the PrimaGIS and PCL community email list:
Maps and PloneGoogleMaps are about georeferencing your existing and new Plone content. They embed tools to geo-annotate (with lat/long) your contents and embed map portlets to view them. Like Eric said, Maps and PloneGoogleMaps can't connect to PostGIS and, as far as I know, can only handle points. If Plone is the major piece of your intranet and you don't have any other spatial data, then these products could serve you well.
PrimaGIS does connect to PostGIS and in the Plone 3 branch will deliver feature data to an OpenLayers map using JSON, but this is probably too heavyweight. Generally, PrimaGIS is a solution when Plone is the major piece of your intranet and you do have other spatial data (shapefiles or spatial databases).
I'm developing a slightly different set of software for my Pleiades project. Plone is just one piece of our system, and we're mainly interested in getting our spatial data out onto the "GeoWeb" so that our community of classicists can view it in Google Earth, Google Maps, or whatever. My new Geographer product for Plone 3 makes GeoRSS and KML views of content which can be handed off to other viewers, perhaps even an embedded map.
Those are the three clusters of spatial tools for Plone as I see them.
If the Maps or PloneGoogleMaps products supported external KML files (which GMaps does), you could stand a simple web service up in front of your PostGIS database to create KML docs from the database. Christopher Schmidt's FeatureServer perhaps. I like this approach in general: an intranet of small apps that can evolve independently instead of doing everything within Plone.
Just for fun, I'll graph the approximate location of the 3 clusters in the space of degree of Plone-centralization of data and Plone-centralization of data interaction/analysis:
(Data) 1 | | zgeo.* Maps | | | | | PrimaGIS | 0 +--------------------- 0 (Analysis) 1
Maps and the zgeo.* software are equally focused on geo-referenced Plone data. Their approaches to presentation and analysis are different: Maps is used exclusively through Plone sites while zgeo is concerned only with the production of documents (GeoRSS or KML) for use in other web applications.