SIP Server is not must. You only need if you need to route calls to SIP end points with dynamic IPs.
For SIP server SER will work well.
There is nothing in PortaSIP that Cisco does not have already.
Actually Porta-SIP cripples Cisco, since it replaces Cisco b2bua role. With Porta SIP you will loose many features in Cisco like:
– SIP-H323 protocol conversion
– Codecs transcoding,
– Calling card IVR,
– CDRs with all Cisco’s VSAs(like voice quality,PDD, etc…).