Yeah, you need to go for the Lumia 720 if you need good battery life. But your budget only gives headroom for Lumia 520. Will you be keeping this phone switched off or in airplane mode when not in use? If yes, then go for the 520. It won't disappoint.
Is there any Android phone with good battery life in this budget? The best I've heard is the Panasonic P51, but it retails for around 19.5k.