How can I create a Views list of all users with either a node OR a comment?
As far as I know you can't create Views for more than one entity type. You would have to create separate Views, one for nodes and one for comments; you CAN however make them appear in the same page by using Views Field View. You can sort each view by author, and get rid of duplicates by using the Aggregation option. 

EDIT:
There IS a way to do two EntityFieldQuery (one for node and one for comment) and then create a single array of authors. Something like this:

$query = new EntityFieldQuery();

$query-&gt;entityCondition('entity_type', 'node');
$node_list = $query-&gt;execute();

$query2 = new EntityFieldQuery();
$query2-&gt;entityCondition('entity_type', 'comment');
$comment_list = $query2-&gt;execute();

$author_ids = Array();

$nodes = node_load_multiple(array_keys($node_list['node']));
foreach ($nodes as $node) {
  $author_ids[] = $node-&gt;uid;
}

$comments = comment_load_multiple(array_keys($comment_list['comment']));
foreach ($comments as $comment) {
  $author_ids[] = $comment-&gt;uid;
}

$author_ids = array_unique($author_ids);
dpm($author_ids);


You can then do whatever you want with the list of uid's - for example load  the users and create a table programmatically for display.