I can remember only a handful of eps where Spock is either called, "Commander," or "Commander Spock." The three that come to mind are Amok Time
(Vulcan Space Central asks for him as "Commander Spock"); The Ultimate Computer
(Commodore Wesley calls him, "Commander"); and in Let That Be Your Last Battlefield
(Bele says, "Are you blind, Commander Spock?" when he makes his comment about why he and Loki are opposed).
It does seem there's a slight amount of informality in terms of addressing senior officers other than the captain, as most rarely refer to Scotty as Commander Scott.
Here's another interesting difference. In The Tholian Web
, when Spock is in command, eventually Scotty, and I think Chekov, call Spock, "Captain," which of course is naval tradition to refer to the person in command of a ship.
TNG was more formal. There are only a few times Picard calls Riker, "Mr. Riker," most notably in A Matter of Honor
. Everyone else calls Riker, "Commander," which is more appropriate given his status as first officer. Data is often referred to as "Mr. Data," and Data often formally introduces himself as "Lieutenant Commander Data."