Double Hungs.

What is a "vertical sliding window"?

Pardon my ignorance of terminology. I consider vertical sliding windows as tall vertical windows that slide horizontally.

The window with the largest clear opening will generate the most circulation, quite simple. Inswing or outswing casements are the best options.

For effective air circulation my preference is tall double-hung windows that can be open at the top and bottom.

Nothing more or less secure about a vertical sliding window than any window for that matter. If the window is open for venting, it can easily be defeated. If your location and demographics dictate that you should be considerate of forced entry potential, you are probably going to want to incorporate some additional security (i.e. bars) into the equation.

While the idea of a tall double hung makes sense from the standpoint of convective air circulation, that's a bit of an overestimation of the truth. Windows on opposite sides of the room where you can benefit from any prevailing winds, will do more to move the air in the room.