Default Underclocking or Hotplugging?

I've been trying to figure out which saves more battery life. Will running a phone on one core while keeping it clocked at.. let's say 1.7 GHz (my phone's stock clock speed) save more battery than keeping all cores active while being clocked at 900 MHz?

Sent from my HTC One using xda app-developers app
My Devices
HTC One M7
Lenovo P700i
Samsung Galaxy Tab 2 7.0

Broken Devices
Sony Ericsson Xperia X1 (Speaker)
PSP Go (Yeah, that's unnecessary. I broke the second battery pack I got because of... valid reasons.)

Coding Knowledge:

I make tutorial videos on YouTube with the help of Marcintosh, my website.