Windows on a Macbook Pro with integrated Intel Iris Pro only
I have no sources other than personal experience to back up my claims, but I recommend the machine without the dGPU if you will be spending the majority of your time in Windows. 

I have seen the difference, and the dGPU kills the battery in Windows. My machine with an integrated GPU gets significantly better battery life under Windows than another machine with a dedicated GPU. In fact, the machine with the integrated chip gets almost equivalent battery life under both Windows and OS X.

You may want to stick with the dGPU if you need it for 3D gaming or media applications that perform better with a dedicated chip. 

In the end of the day, it is really up to you, but you will probably get better battery life without the dGPU. I have seen an hour and half difference in some cases.