For windowed, because the resolution you render to is normally proportionate to the physical size of the window. So if you needed to render to something weird like 520x370 to get an accepetable framerate, how would you handle that? In windowed mode, you'd end up with a tiny window. In fullscreen mode, you'd probably have to drop to 320x240 or go up to 640x480. In both cases, if the game's HUD or text isn't designed for such low resolution, you're SOL (notice in those pics that the text and HUD is still sharp.. it's just the scene that's lower res).Why do you need to switch to a larger resolution? Or rather said... what gain is there from something weird like let's say 520x370 or something like that?
You gain not having to run complex pixel shaders as often (196,608 times for a full scene in 512x384, versus 786,432 times in 1024x768; not counting overdraw or any other offscreen rendering). And if you're running on something like a GF6 (which is fully DX9/SM3 capable), or even a GF7 or lowend GF8, removing that extra processing could be the difference between a playable framerate, and a bad gaming experience.