With WinForms, which method is more efficient, close and reload or hide/show?

Personally, if I don't need a form, I close it. I'll just reload it if ever I will need it again. Generally there's no need to put it in memory, unless you really need to pick up some date from that form behind the scene.

With the hardware these days, loading times for winapp forms are really fast. And think for example if you have 100 forms, it will really be troublesome if all those forms were only hidden and not closed. It will really eat up a lot.

Just my two cents.

There is not big difference in the memory you'll save.

Try it out.
 
Start your app and open task manager. Watch your memory usage. Now hide a form and notice the memory usage again.

Now do the same thing, but close the form instead.