nunya's comments are good. But would you like to know more?....
Red dot is target focused (your eyes) at all times. If you have trained your index correctly, it is much faster and more accurate. If you don't train your index, it's pretty easy to lose the dot.
The larger windows are nicer to use, but it doesn't really make a performance difference assuming you don't lose the dot.
If the back side of the reflex glass gets misted over, or a water drop in the wrong spot, it is possible that the dot will stop working (doesn't reflect correctly). That's where the various enclosed emitter dots come from.
Actual suggestions: Setup your pistol with back up irons that co-witness in the lower 1/3 of the window. They're useful for finding the dot as you learn how to use the setup, and a backup if the glass gets foggy/misty. Practice the draw to presentation/index, A LOT.