I never played in Windowed mode. Always full screen :)

Some games I like to play in Windowed mode.
I can write down notes, use the calculator, a walk thru
or a number of other things while playing the game.

I like the older adventure games that are slower paced but
look horrible on a modern hi resolution screen. The smaller
windowed mode looks much better

So I guess Full screen: modern games
Windowed: Older games

How's that for a "politicians stand" ....

For those games you can turn on GPU scaling and set the scaling to Center so it doesn't stretch the image. This way in fullscreen the game will be rendered in its original resolution in the middle of the screen, the rest of the screen will be black.

Do you need an external video card?
I just have the Surface Pro 3 integrated Intel

I think Intel does have that too, check in the Intel GPU control panel, it should be there somewhere, look for "Display scaling" or something. I could swear I had to use it once when I connected a Intel laptop to a TV.

Always fullscreen.

Fullscreen unless I'm streaming, then I usually game in windowed.

I am minority here.
I love to play games on windowed mode.

Unless it's a casual distraction like a card or puzzle game, I always play in fullscreen (borderless window fullscreen when possible).

99% Fullscreen, because Windowed and Borderless Windowed mode usually come with input lag.

To follow up on my earlier statement: Fullscreen seems to use less system resources than windowed, and for games like FF XV, every little bit extra I can get, makes a difference.

Fullscreen always.

I have two monitors, but I only use them both for work. If I play an oldish game I`ll switch to one and go Fullscreen cos I always like Fullscreen.

Full Screen 95% of the time. Sometimes if it's a low res/retro game that looks bad in full screen, I'll play in a window.