iPhone 6 user here, I was due an upgrade in Sept but have decided to hold out for the next new model, the iPhone form factor hasn't really changed that much during the 6/6S/7 incarnations, rumours are (& it's just that) that the next iPhone will be something special to coincide with the 10th anniversary year of the iPhone.
For now I've just gone on a SIM only deal whilst I wait, if you are happy with your iPhone 6 plus then if it were me I'd stick with it.