Yes, there are differences. One of the big differences is that third party SIP providers like EtherSpeak, have the ability to support applications for various voice infrastructures that require certain accommodations associated with the PSTN. Unlike a larger main carrier, third party providers are more flexible in these matters and have the ability to get customers up and running in a fraction of the time and cost it would take a larger carrier.
In addition, third party SIP providers handle the enforcement of real time policy, routing, control, monitoring and interoperability of the individual PBXs into a common SOA based session layer. By eliminating direct point to point communications between clients and servers, organizations are better able to leverage their cost savings due to this type of vendor consolidation. This even includes services like fax-over-ip, video-over-ip, etc. Here is a case study found on the ShoreTel website which is a good example of the advantages of using third party providers vs. the larger carriers: http://www.shoretel.com/about/newsroom/press_releases/2010-04-27.html.