iMessage or Text Message?

I currently have an iPhone 5s and use it for calling and texting purposes without data. I also have an iPad Mini 4 and an iPod touch 6th Generation that I use iMessage to text other people who have Apple iDevices. When I leave iMessage off on my iPhone, every message is sent as a text message and if I turn it on, the iMessage will only send when my iPhone is connected to wifi like my iPod and iPad. Is there a way I can leave iMessage on so that the iPhone will use iMessage whenever it has wifi and automatically use text message whenever I lose wifi?