Chances are that the window is a single hung window and the top pane does move. Double hung windows are usually more expensive and since you live in an apartment they most likely had single hungs installed to save money.
If you don't have a problem with energy efficiency in that one room, then I would just go with single pane. Double pane units don't last forever. The argon gas will eventually leak out and then you have to pay to replace the glass.
