Let's now return to the text of John 3:13 in Houghton's textual commentary. For the data, go here. As he did with Matt. 5:22, Houghton seems to see-saw back and forth, skipping from arguments for the longer reading and then arguing against the longer reading. On the one hand, we're told that the longer reading has wider attestation; it was possibly omitted due to scribal oversight; and it may have been found objectionable based on its theology (can the Son of Man really be in heaven and on earth at the same time?). On the other hand, it's argued that the longer reading isn't found in the earlier witnesses; it may have been "attracted" to the two previous mentions of heaven in the verse; and it is omitted in the SBLGNT as well as the THGNT.
His conclusion? "External evidence leads to the preference for the shorter reading here ...."
Though he doesn't quote Metzger, Houghton's remarks hearken back to his predecessor's suggestion that the longer reading is "an interpretive gloss, reflecting later Christological development." However, as I argued in my GTS essay on John 3:13, the longer reading, which indeed may have possibly originated as a scribal addition, was more probably omitted in the Alexandrian Family of witnesses to relieve a perceived theological difficulty. It's also very much in keeping with Johannine style and theology.
Once again, we see internal evidence used like a wax nose and twisted to support different readings. Internal arguments have been used in multiple ways, and this in my mind raises the issue of subjectivity. By way of contrast, a theory based on external evidence deals more with objectivity. To me, the external evidence solves the problem. The internal evidence (which can often go both ways) corroborates the solution.