1. Battery life. I remember fitwxc said that change transport from UDP to TCP extends battery life.
2. Call quality on Wifi, 4G and 3G, respectively. I remember fitwxc said that any client using XMPP to connect to GTalk won't have stable sound quality on 3G, because of G711 bandwith requirement.
If all are good then one time $5 is a great value. You don't need complicated SipSorcery stuff.