well i think webmin and ispconfig are 2 completely different things ... except for the web interface
ISPConfig is more for the, say .. junior system-, networkmanager, webmin for the senior ... no wait, a senior would just go console
Webmin was the first one that came up to me that has a web interface and has postfix support. Don't recall ispconfig has this functionality.
Actually, that should be very nice, if ispconfig will get some sort of "Expert mode" which enables all these extra functionalities, like mailqueue monitoring f.e. some stuff that webmin has