They look different.
Drop down lists have an "arrow" option not found for pop up menus.
Drop down lists can be set up as auto-complete lists where you enter several characters and the list filters down to just values that start with those characters.
When "only show values from second field" is specified, the second field value remains visible in the field with a popup menu format when focus leaves the field even though the first field value has been entered. For a drop down list, the first field value appears when you exit the field.