Geoserver and Mapserver are both quite powerful but their developers pursued different goals.
To make the choice easier for you (I hope), here comes a general feature comparison:
|WMS||both are good||maybe a bit better |
|WFS||better, supports WFS-T ||no WFS-T |
|Technology||J2EE ||CGI |
|Project start||2003 ||1996 |
|Administration||Web tool||Mapfile generation can be aided by QGIS, but not comparable to Geoserver’s web admin tool|
|Extensibility||good for Java developers ||PHP Mapscript, good for PHP developers |
|Cartography||uses standardized SLDs||powerful; styles are part of mapfile|
|Services||one WMS/WFS/WCS service for all users ||a mapfile means a service |
|Querying||CQL and OGC filters||embedded SQL statements|
New benchmarking results should be available soon . Meanwhile, you might wanna watch last years results .