In my experience, use BlazeDS if you don't need to use web services that various technologies can access.
BlazeDS
Pros: less intensive server, less client processing time, less data packet (it is binary), which means that overall a quick call. It can publish / subscribe, as well as call a method.
Cons: Not compatible with external ends of non-Flex (although it's open source, so theoretically it could be.)
Webservices
Pros: well established, almost cross platform. Easy to read and translate problems.
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