An app running in the background will not reduce battery life (unless it is designed to actively do stuff in the background).
That's why task killers are not encouraged withthe Android OK. Just because the app is loaded into the memory doesn't mean that it is consuming battery. In fact, when it needs to be active, it will relaunch (or you will launch it), and now, it will consume (slightly) more battery because it now has to load back into active memory.
Regarding your question, the application decides what the Back button really does.
Some apps will just suspend when you hit the home key while in the app, and only "close" when you hit back. Others will close when you hit home. Some others will not close even if you hit back or home.
My recommendation: Use whatever you prefer. Let the OS handle the suspend/close