There seems to be two contradicting conventions on web applications. One is that a Dropdown-control also is an action button, meaning that when a user selects an item in the Dropdown control, an event fires and you get to the next step imediately.
The other one is the same as the above except for the action which is moved to a simple button next to the Dropdown control. The workflow here is select, then action. A two step activity instead of just a single step.
Which one is the preferred one?