When does the discrete graphics card get utilized in the 15'?

[questionwonder]

I'm trying to decide between the $1999 13' TB with 16RAM vs the $2400 15' base model and I want to know when the discrete graphics card in the 15' gets turned on and utilized? The reason I want to know is because I want to get a second monitor (4K or 5K) and if the graphics card isn't needed or doesn't get turned on just for a screen or 2 screens connected, then I might not consider the 15' because the 13' has an Intel 550 internal graphics card, while the 15' has only a 530! If what I'm using the machine for doesn't require the Radeon graphics card then I want the machine with the better Intel internal graphics card. I don't do any video editing or processing.

My current setup and use.
I'm use my machine for browsing the web, watching some movies I download and I run VMWare Fusion with Windows 10 running Visual Studio 2017 RC to do all my development. I have a 22' Samsung 1080P monitor I connect to my current mid 2013 Macbook Air 1.7 Gz, 8Gb RAM, 500Gb SSD.

 

[poorcody]

I've got a 15" and as soon as I hook-up my Thunderbolt Display, the discrete GPU kicks on and stays on. Honestly don't know if it's needed, but it does it automatically.

ADD: By the way, I run Visual Studio under Windows 10 as well, but using Parallels. I notice Parallels does not start the dGPU.

 

[R3k]

Used when connecting a 2nd display, gaming, or using pro apps like final cut.

Otherwise it just uses the iGPU.

Of interest, there is an app that forces the mac to use the dGPU for everything, but I never noticed a performance increase in day to day use.

Edit: so yeah, it'd be helpful to have in your usage scenario.