You have a bad iPhone then. I can get 6-7 hours of use off a full charge. I couldn't even get 3 out of the Noteand unlike the other ridiculous claims, here is the proof...
Why should he prove a god damn thing to you? This is a thread about iMessage and nobody gives a **** about whether you believe his battery life.
As to your question, unfortunately with iMessage it all gets kind of blurry. Once you start using it with anything you are kind of stuck. I wish they had made it iPhone only since then you could actually use it like SMS, but since its on the ipad and ipod touch you have interesting problem where if you are out of a data area you can get an iMessage and it correctly says delivered because it is sitting on your iPad at home, but you never got it on the phone that you actually have with you.