RE: Howls or Distress?
The problem with using only one or the other is, they often don't vocally respond to a howler but yet they will come to it silently, just like a distress, only usually slower! SO if you do some howls and don't here anything and move on, you're probably driving/walking away from incoming coyotes and don't even know it!
Lots of guys, including myself, use both a howler and distress on sets, especially this time of year. It works! If you do some howls to start a set, wait several minutes before going to distress. I usually plan on staying longer on a set if I used any howls. Female invitation or interrogation both seem to work well this time of year!