I think disadvantage of Snom (or the standalone ser) is that this solution is not integrated with the billing. May be it’s ok for the small company when you want to establish connection between the offices, but you definitely need more if you want to provide SIP-based commercial services. With PortaSIP+PortaBilling you can start doing your business within 1 hour after the installation.
P.S. IPTEL’s ser is a part of PortaSIP, and Porta Software developers are contributing to it’s development.