Eden, here is a few opinions of the Committee responsible for the Standard text, used in most modern translations. Sorry about the fonts.
3:13 {B}
On the one hand, a minority of the Committee preferred the reading ?νθρ?που ? ?ν ?ν τ? ο?ραν? , arguing that (1) if the short reading, supported almost exclusively by Egyptian witnesses, were original, there is no discernible motive that would have prompted copyists to add the words ? ?ν ?ν τ? ο?ραν? , resulting in a most difficult saying (the statement in 1:18, not being parallel, would scarcely have prompted the addition); and (2) the diversity of readings implies that the expression ? υ??ς το? ?νθρ?που ? ?ν ?ν τ? ο?ραν? , having been found objectionable or superfluous in the context, was modified either by omitting the participial clause, or by altering it so as to avoid suggesting that the Son of Man was at that moment in heaven.
On the other hand, the majority of the Committee, impressed by the quality of the external attestation supporting the shorter reading, regarded the words ? ?ν ?ν τ? ο?ραν? as an interpretative gloss, reflecting later Christological development.[1]
3:13 ?νθρ?που (of man) {B}
The words ? ?ν ?ν τ? ο?ραν? (who is in heaven) at the end of the verse are most likely an interpretative comment by a copyist reflecting later Christological development. On the other hand, the longer reading may be original since the shorter reading is supported almost entirely by Egyptian witnesses. And the diversity of readings may imply that copyists found the expression ? υ??ς το? ?νθρ?που ? ?ν ?ν τ? ο?ραν? difficult and changed it either by omitting the participial clause, or by changing it so as to avoid suggesting that the Son of Man was at that moment in heaven. The shorter reading is found in most modern translations (for example, NRSV and NJB), but a few follow the longer reading (REB, Seg).[2]
{B} {B} The letter {B} {B} indicates that the text is almost certain.
[1] Metzger, B. M., & United Bible Societies. (1994). A textual commentary on the Greek New Testament, second edition a companion volume to the United Bible Societies' Greek New Testament (4th rev. ed.) (174–175). London; New York: United Bible Societies.
NRSV New Revised Standard Version
Seg Segond
NJB New Jerusalem Bible
[2] Omanson, R. L., & Metzger, B. M. (2006). A Textual Guide to the Greek New Testament: An adaptation of Bruce M. Metzger's Textual commentary for the needs of translators (169). Stuttgart: Deutsche Bibelgesellschaft.