I am assuming you have to buy a mailbox license to activate the embedded VM system as your voicemail is probably on NuPoint.
I would buy a mailbox anyway, it doesn't hurt to have the embedded RADs to utilise.
The advantage of using the IVR ports is that you can have different RAD greetings using the same port, whereas the embedded you have to dedicate a port per greeting.
If you don't have many greetings I would just go with embedded.