You can configure your Nexus 5, and specific apps and services on it, to not use mobile data in the background. In Settings -> Data Usage scroll down and you will see apps. Select the app, scroll down again and you will see the setting "restrict background data". Check this box and it will stop using mobile data while the app is in the background. This is especially critical for things like Google+, where if you take one HD video it will try to sync it in the background and kill your monthly quota. My kids were making videos with my phone while I slept...
Ting looks like a nice deal, and enthusiastic support. I was very frustrated dealing with Sprint to acquire a SIM for my Nexus 5. They were abhorrent in person at three stores and in online chat as well. That kind of put me off. I did finally manage to get SIMs from Ting real easy like. But the experience put me off even of using Sprint towers.
Besides, T-mobile has been really nice to me. That's worth a few extra bucks.
Stay away from Microsoft Skype on your Nexus 5. It has a known issue that kills the battery. Imagine that.