A bishop always has the preference to be the Celebrant. In this case when the Bishop defers the position of Celebrant, he is to be vested in a similar manner as the Celebrant unless he, again defers. While a crosier may only be used by a bishop who is the Ordinary of the dioceses unless given dispensation by that Ordinary, a mitre may always be worn. Within religious orders, a Father General, Delegation Superior, or Provincial Superior also has the right of deferral of the Celebrant like the Bishop and is vested in the same manner but simply does not wear the marks of episcopal office since he is (usually) not a Bishop.
I think it should be noted that there is a difference between clergy choir dress and cleric choir dress. In some Clerical Orders like my own, the use of minor orders are still used within the formation and as such the distinction between clergy and cleric still exists. Clergy are ordained and clerics are those who have taken vows who are in formation to the priesthood and who have already been ordained. Cleric choir dress is simply a habit or cassock with surplice, whether they are simply in formation or already priests. This can be for the Liturgy of the Hours or for clergy and clerics who are attending Mass within the choir and are thus not concelibrating. Clergy choir dress includes the stole and is for concelibrating clergy who may or may not be in the choir and for liturgical and sacramental functionary outside of the Mass like Baptisms, Rites of Committal, and non-sacramental blessings.
Concelibrating priests may wear the chasuble in the OF, but it must be plainer than the Celebrants and the stole is often worn over the plain chasuble instead of under. This is more common with large numbers of concelibrating priests to differentiate the principle concelebrants from those who cannot fit in the sanctuary.
The actual cassock (or habit) is usually the same whether or not in choir dress or not.