Which is more efficient?

[Admins]

If you hide it and show it again later then the position and size are the same, which is often better than recreating it and the user needs to move it again if it's not at the good place. You can use showtop() to make the window appear on top, un-minimize and get the input focus.
Normally creating and hiding windows is not a big problem, they might take a few kbytes in memory but should not be a speed problem. There are many default windows (playerlist, F5, F2 etc.) which are created in invisible state when you login to the login server. For stuff like PM windows it needs to create new windows of course, and is then also destroying them when you close them.
So I would say if you only need the window one time then destroy it afterwards, and if you plan to open it several times then just hide and reopen it next time.