For what it's worth, here's what I remember in practice (based on my experience at Juventutem):

For Bishop Rifan's Solemn High Mass (extraordinary form) at Ottobeuren Abbey, the schola was made up of males. Mass was celebrated at the lower altar [photo]. We sang in the sanctuary between the choir stalls and before the high altar [photo]. We would process into the sanctuary, sing the proper chant, and then exit the sanctuary. There was another choir (mixed children's choir plus soloists) in one of the lofts that sang the Mozart Sparrow Mass (K220). And finally, a third choir (also mixed) outside of the sanctuary and in front of the congregation, sang the processional hymns and the Marian antiphon [photo 1, photo 2]. As you can see from the photos, the men wore blue robes, while the women wore blue capes. All wore black underneath. The choirmasters were dressed in an additional garment (whose name escapes me).

(The fellow you see wearing sneakers is only doing so because his luggage was lost - he received his bags later that day.)

No answers from me as far as the ordinary form goes, but I will note that as far as most churches are configured, the choir will not be singing from within the sanctuary. Corrections are most welcome.

For the ordinary form of Mass, the brand new US bishops' document on music says:

33. Choir and ensemble members may dress in albs or choir robes, but always in clean, presentable, and modest clothing. Cassock and surplice, being clerical attire, are not recommended as choir vesture.
http://www.usccb.org/liturgy/SingToTheLord.pdf

In the extraordinary form, I believe men and boys who sing the propers are considered to have a clerical role, and so may be vested in cassock and surplice, and may sing from the sanctuary. I believe one of the documents from Pope Pius X or Pius XII mentioned this (I'm not sure which). In my opinion, for either form of Mass, cassock and surplice are considered clerical attire, which women should not use.

I think it's really hysterical that the bishops see cassock and surplice as "clerical," when none of them every really utilize "choir attire" in the traditional sense. When did you ever see a priest (a typical Vatican II priest, that is) wear a cassock under his vestments, or anytime he was at the church?

The surplice, as I understand it, is simply a variation of the alb. They all wear albs, or worse, cassock-albs, which really is a clerical garment (despite that chirpy little notion that the alb represents our baptismal garment . . . yah, whatever) that altar girls are permitted to swish around the altar clad in.

Meanwhile, the typically Protestant (and usually butt-ugly) Wesleyan-style choir robe, which usually features some kind of stole (a decidedly clerical vestment) is recommended! Note that nothing whatsoever is said about proper color or design.

EF or OF, I say dress the part, with the dignity the liturgy and the ministry of musician deserves.

I'm not saying that our Anglo-Catholic brothers and sisters have a lock on any of this, but at St. Clement's Philadelphia the men wear black cassock and white surplice, the women wear black cassock only with a black zucchetto (I think that's the proper term, it's a black scull cap), and the men who are serving in the role of cantor (precentor) for the chant portions wear copes. (That was their custom anyways, back in the '90's).

I'm not sure if this is at all in line with proper Catholic usage, although I'd bet dollars to draughts that if one were to consult Fortesque, this would be considered the norm.

As long as there's no hard and fast rule:

White linen albs for everybody!

Even for the conductor. Chorus angelorum, like. Neutral.

One organist I know at a cathedral wears a black cassock. Given his personality, we can't imagine him in anything else.

I have from my bishop an original draft review copy of the "Sing To The Lord" document, which states that "...Cassock and surplice, being clerical attire, are not appropriate as choir vesture. " Since the original wording was officially modified from "not appropriate" to "not recommended", I would infer that lay vesture in cassock and surplice was determined to be not completely out of the question. I suspect the change in wording is in recognition of the fact of respected liturgical choirs (for example, the Cathedral of the Madaleine in Salt Lake City) already being attired in cassock and surplice. In any case, it would appear that the question is still open, at least until Rome weighs in on it.

I believe that GIRM says that any ministers who do not wear outer vestments--and this includes all lay minister--may wear the surplice instead of the alb. The surplice is nothing but a form of the alb that was designed to be worn over fur in cold churches (super + pelliceum) . By what reasoning would the bishops say that the surplice is clerical attire but that the alb is not?

There's always room for another angel on the pin head.