Prefer the sort_by method over the sort method whenever you provide a block to define the comparison.

Common form:

line_adds.sort { |x, y| x.elements["ItemRef/ListID"].text <=> 
  y.elements["ItemRef/ListID"].text }

Preferred form:

line_adds.sort_by { |x| x.elements["ItemRef/ListID"].text }

For small collections both techniques have similar performance profiles. When the sort key is something simple like an integer there is no performance benefit from sort_by.

The performance difference is especially noticeable if the sort key is expensive to compute and/or you have a large collection to sort.

The algorithm that yields the performance benefit is known as the Schwartzian Transform.