Error message sent to browser upon runtime errors, or not. Default error screens are ugly, roll your own if you don't like them. You can trap and handle errors in script, although the process is ugly and limited. Allows specification of a default error handling HTML template, into which error information is plugged, much easier to display consistent and complete error information. No ability to trap errors in script and take conditional action.
All errors abort script processing and display the error template.
Universal server support, especially in the UNIX world. ASP is also making huge strides quickly. Plenty of online support. Microsoft does not support ASP and even their com and Dcom objects are not really industry standard. There is some free support from Allaire via their Forums.