Hi Steven. No, I have not switched my preference in translation but Metzger's translation is equally valid. Both convey the same meaning, but emphasize different nuances of the Greek.
It's definitely not arbitrary, cjab. What I said is that Koine Greek prefers a participle to a relative clause. As I have noted in the other thread, while (I should really say
roughly) equivalent in function, an attributive participle is primarily
restrictive, whereas the relative pronoun is primarily
nonrestrictive. In other words:
A restrictive clause restricts or defines the meaning of a noun or noun phrase and provides necessary information about the noun in the sentence. It is not separated from the rest of the sentence by commas . . . A restrictive clause is also sometimes referred to as an essential clause or phrase. (source: Waldenu.edu)
The attributive participle be used to limit/restrict, specify, or identify their antecedent. The participle can operate like an adjective and yet it can also govern objects, be modified by a prepositional phrase, etc. This is true of the class of attributive participles, not just ὁ ὢν.
I didn't exactly say "interchangeable," but both times it means "who is."
New Testament
It also operates relatively in more than two places in the
whole of the NT: John 1:18 (ὁ μονογενὴς υἱός, ὁ ὢν εἰς τὸν κόλπον τοῦ πατρὸς), 3:13 (ὁ υἱὸς τοῦ ἀνθρώπου ὁ ὤν ἐν τῷ οὐρανῷ, as found in nearly all manuscripts), 12:27 (ὁ ὄχλος ὁ ὢν μετ᾽ αὐτοῦ), 2 Cor. 11:31 (ὁ θεὸς καὶ πατὴρ τοῦ κυρίου ἡμῶν Ἰησοῦ Χριστοῦ οἶδεν ὁ ὢν εὐλογητὸς εἰς τοὺς αἰῶνας).
There are also several examples in Revelation: 1:8 (ὁ κύριος ὁ ὢν καὶ ὁ ἦν καὶ ὁ ἐρχόμενος ὁ παντοκράτωρ), 4:8 (ὁ θεὸς ὁ παντοκράτωρ ὁ ἦν καὶ ὁ ὢν καὶ ὁ ἐρχόμενος), 5:5 (ὁ λέων ὁ ὢν ἐκ τῆς φυλῆς Ἰούδα), 11:17 (ὁ θεὸς ὁ παντοκράτωρ ὁ ὢν καὶ ὁ ἦν). However, John's usage is somewhat idiosyncratic (Cf. 1:4 χάρις ὑμῖν καὶ εἰρήνη ἀπὸ ὁ ὢν καὶ ὁ ἦν καὶ ὁ ἐρχόμενος; 16:5 δίκαιος εἶ ὁ ὢν καὶ ὁ ἦν ὁ ὅσιος (NA/UBS)), so it's harder to form conclusions.
Old Testament
In at least two instances in the Greek OT ὁ ὢν is used where we find a relativizer (אֲשֶׁר) in Hebrew:
הָעָם אֲשֶׁר אַחֲרֵי עָמְרִי (1 Kings 16:22)
ὁ λαὸς ὁ ὢν ὀπίσω Αμβρι
The people who are following Omri (=The followers of Omri)
אֶֽהְיֶה אֲשֶׁר אֶֽהְיֶה (Exodus 3:14)
ἐγώ εἰμι ὁ ὤν.
I am Who Is
As I have said elsewhere, attributive participle can be used substantively when the head noun is implied.
Inscription
For an example outside the GNT and GOT, it also occurs in an ancient funeral inscription of Abercius, bishop of Hierapolis in Phrygia, from about the mid-2nd century:
οὔνομα Ἀβέρκιος ὁ ὧν μαθητὴς ποιμένος ἁγνοῦ (Inscription of Abercius)
My name is Abercius, who is a disciple of the holy Shepherd
Translation of the Attributive Participle as a Relative Clause
As every grammar within my reach will tell you (I noted A.T. Robertson, Funk, I think also Mounce, Wallace elsewhere so will not repeat them here),
An attributive participle should normally be translated with a relative clause (e.g., “the Father who sent Him,” τὸν πατέρα τὸν πέμψαντα αὐτόν). (Köstenberger, Andreas J.; Merkle, Benjamin L; Plummer, Robert L.. Going Deeper with New Testament Greek, Revised Edition, p. 327).
The best way to translate an attributive participle is by means of a relative clause. A relative clause is one that begins with a relative pronoun (“who,” “which,” or “that”). (Black, David Alan. Learn to Read New Testament Greek, p. 150).
These all fall well within the range of usage of the attributive participle, the same rules hold true with other attributive participles, and frankly I've never seen this or the usage of the attributive participle functioning like a relative clause disputed, as you and The Real John Milton have disputed it. There's no reason to treat ὁ ὧν as an exception to the rule.