I would do both, as well as disabling SIP ALG.

You don't really want the IP address of your GS changing, as that can only cause issues. I would port forward as well, as you don't want anything sent back and forth on those ports clipped for any reason. While you at it, you probably want to disable SIP ALG as well.

Just my opinion, and I'm no VOIP or TCP/IP expert, nor do I play one on TV.