nextElementSibling
always returns an element. nextSibling
can return any kind of node. They are the same for your example, but different in other cases, e.g.:
<p><span id="span-01">Here is span-01</span>
Some text at the top level
<span id="span-02">Here is span-02</span></p>
In this case, document.getElementById('span-01').nextElementSibling
is span-02
, but document.getElementById('span-01').nextSibling
is the text node containing "Some text at the top level" (or, as pointed out by @Manngo in the comments, the whitespace that separates that text from the element above -- it seems some browsers put whitespace between elements and non-whitespace nodes into separate nodes, while others combine it with the rest of the text).