If you keep switching data and WiFi on & off all the time, most likely you're going to use more battery than leaving it on. You'll end up having your screen more time on (that's the battery user number 1) and your processor will be working an extra time. These are my basic rules.
- Use WiFi whenever available. Install some app or make your own shortcuts to toggle WiFi and mobile data with as few clicks as possible.
- You can leave WiFi and mobile on at the same time, after lollipop your phone will switch to mobile when no WiFi is available (and back). In your settings go to WiFi - advanced and uncheck network notifications.
- Try to avoid mobile data when you're in an area with bad signal. That will drink a lot of your battery juice.
- In your apps check the sync settings for mobile data. Keep them to the minimum that suits your needs. Apps like Facebook, Google +/photos, news readers, etc could destroy your data plan if you don't change those settings.
- Widgets are nice, but you don't want the weather info refreshing every five minutes, specially on mobile data. Same with news reader and so on.
- If you like reading in your phone use an app for offline reading, such as Pocket.
- And, of course, if you don't need to be online you can always switch the whole thing off
Look. I've got a veeeery cheap data plan with my prepaid SIM card. It's just 200 MBs a month. That's enough for my needs and I'm online most of the time. And I can get more than 5 hours SOT with no problem. So, you don't need to worry too much about these things