Note: This isn't a question about which icons or labels to use. It's purely about usability.
So, I have a page with expandable panels and I want to add a button to expand/collapse all panels. Now, I have 2 options, but I'm wondering as to which one is more usable.
[ Expand all ] [ Collapse all ]
Pro: It's quite obvious which button does what
Contra: 2 buttons for a similar action seems somewhat overkill
[ Expand / Collapse all ]
Pro: Just one simple button
Contra: The state isn't shown anywhere. This isn't usually a problem when either all panels are expanded or collapsed, but it becomes an issue when some panels are expanded and some aren't.
So, which one seems more usable?