Hangouts isn't anywhere near as integrated as iMessage. Yes, it does do SMS and internet based messaging, but not seamlessly. Or I can't get it to. The SMS contact is distinct from the google contact. So you send it one or the other.but you can't just have a hangout with someone and then if you send a message when they are out of wireless or 3g it falls back to SMS. With imessage it sent wireless OR sms without you knowing. With hangouts you need to choose if you send it wireless or sms
Or at least I haven't figured how to get it to.