using echo when filtering the_content

Is it a sin, or at least bad practice, to use “echo” in a function that has been added as a filter on “the_content” ?

function myFunc($content) {
  echo "This gets printed first";
  return $content;
}
add_filter ('the_content', 'myFunc');

Or is the only accepted way to modify the incoming content string:

function myFunc($content) {
  $content = "This gets printed first".$content;
  return content;
}
add_filter ('the_content', 'myFunc');

?