Letās break this down. First a cassock is not a āvestmentā it is (or was) everyday garb for priests. A surplice (particularly the waist-high cotta commonly worn by servers in the past) is a far less formal substitute for the alb. While the cassock/surplice was commonly worn in the past itās by no means more formal or even more appropriate than an alb.
At some point the Church began to allow laypersons to wear albs when assisting at the Mass. This came about for a huge number of reasons, not all of which were good. Probably the principal one was because the alb is the garment of our baptisms. It took took a long time to allow the wearing of albs because I suspect (depending on the person and the alb) that it can lead to confusion over who is ordained and who is not.
Personally I think albs look a great deal better. Young servers can look just fine in cottas and cassocks but I think adults who wear them can tend to look like posers.
Cassock and surplice isnāt as formal, but it doesnāt always make sense to wear something of highest formality.
A priest in a liturgical function wears sometimes cassock/surplice/stole or alb/stole, either with cope sometimes, and for mass puts on alb/stole/maniple/chasuble. Subdeacons wear tunicles, deacons wear dalmatics, though they also wore folded chasubles at other times.
Anyway, the point is that simple cassock and surplice, which is less formal, emphasizes that the clergy assisting were only assisting (or only in choir) and not serving as a celebrant or sacred minister.
I think albs are fine for servers, thatās custom in some places, but the less formal cassock/suprlice makes sense too.
Frankly none of it really makes sense for girls. Cassock/surplice is clerical, so it should be out.
Albs have become what clerics wear under tunics/dalmatics/chasubles. Yes itās our baptismal garment, but to pretend that anyone actually wears it after baptism except for the ceremony itself is archeologism. Do the altar girls wear the alb before and after Mass too, since they are baptized? Of course not.
But if you have to have something, I suppose alb is best.