I'm using a Pi Zero in a portable project and wondered if there were any power savings to be made by using an SPI display over a HDMI one or vice versa.
I think MUCH larger difference will be in how much power the display uses based on its size and type rather then the control signals you are feeding it with. An 20 inch LCD monitor over HDMI is going to draw way more power then a small SPI screen. And if you are working with the same size/type of screen. I still think the large draw for the screen will make the small difference in power consumption between the two types of communication.