The Toggle Comment command in the Block menu combines the functionality of Block|Comment and Block|Uncomment. On each line in the selection, it attempts to remove the comment characters exactly like Block|Uncomment does. If it does not remove any comment characters from a particular line then it adds comment characters to that line just like Block|Comment does.
If only one line was selected or if there was no selection, Block|Toggle Comment moves the text cursor to the next line. This allows you to quickly comment and uncomment multiple lines without having to select them if you assign a keyboard shortcut to Block|Toggle Comment.
The advantage of Block|Toggle Comment is that it allows you to use the same keyboard shortcut for commenting and uncommenting lines. But if you need to comment out a larger block in which some lines are already commented out that need to stay commented out, then you need to use Block|Comment.