Using wi-fi will keep your data usage down but it can also suck the life of your battery. I turn off the wi-fi on my phone unless I am actually using it. My battery would rapidly head toward shutoffville until I started doing that.
Granted, I have an iPhone, and Apple has made concerted efforts in terms of battery life, but I generally leave WiFi on all the time.
On the occasions when I know I need to stretch the battery as long as possible (such as recently at all-day baseball tournaments for my eldest), I generally disable Bluetooth, WiFi, and location services. (The drawback on the last one is losing geo-tagging on photos, but sometimes tradeoffs must be made.)