Which one abides by the current standards and would be the logical place to look
Well it is question of point of view: imho the most logical and immediate place to look is between the two (or more) posts that need to be merged. But it really doesn't matter if the feature has a voice into the moderator menu to be accessed and used. It could be ok too.
But imho my solution is faster than the menu. With the menu the staff member that want to merge two posts into one have to:
Scroll the page to go to the menu
Click onto the menu
Then click to the voice of the menu
Click to the button to confirm the choice made into the menu
Wait the new page to load
Choice from the list the posts to merge into one by clicking onto two or more checkboxes
Scroll the page till the button to confirm
Click on the button to confirm the operation
Wait the confirmation page to load
Click on yes to confirm the operation.
I hope that I have not forgotten any step.
With my solution:
Click the merge button and (eventually) confirm the operation.