The Girl Group Sound: The Darlings Of The 1960's series of
compilations on Sha-Boom is half way between the One Kiss Can Lead to
Another: Girl Group Sounds Lost and Found boxset
on Rhino and
The Girls of Hideaway Heaven on Rare
Rockin' Records because it combines some rare and hard to find girl
group records with some easier to find sides.
This series is more true
to the "girl group sound" then the Rhino box (which took jaunts into
the girl soul and girls in the garage sound along with the classic
girl group sound), but has poorer bootleg like sound quality (the
Rhino box had impeccable sound quality for recordings so old), and
doesn't dig as deep and as obscure as the Rare Rockin' series.
The Girl Group Sound series seems like it was a blueprint for the
Rhino box (they were released in 2003, while the Rhino box came out in
late 2005), where some titles that could not be licensed were replaced
with a wider spectrum of genres and solo artists. There's enough
difference between each of these though that true girl group
collectors won't be disappointed when picking up any and all of these
collections. There's bound to be some titles you haven't yet heard.
The hard part will be finding these now, since they're not available
through the major retailers. Your best bet is to find it through the
specialty shops or auction sites. But there's enough hard to find
titles here that would make searching these out worth it, if you don't
mind the bootleg quality and lack of liner note information.
The Girl Group Sound: The Darlings Of The 1960's series of
compilations on Sha-Boom is half way between the One Kiss Can Lead to
Another: Girl Group Sounds Lost and Found boxset
on Rhino and
The Girls of Hideaway Heaven on Rare
Rockin' Records because it combines some rare and hard to find girl
group records with some easier to find sides. 
This series is more true
to the "girl group sound" then the Rhino box (which took jaunts into
the girl soul and girls in the garage sound along with the classic
girl group sound), but has poorer bootleg like sound quality (the
Rhino box had impeccable sound quality for recordings so old), and
doesn't dig as deep and as obscure as the Rare Rockin' series.
There's enough
difference between each of these though that true girl group
collectors won't be disappointed when picking up any and all of these
collections. There's bound to be some titles you haven't yet heard.
The hard part will be finding these now, since they're not available
through the major retailers. Your best bet is to find it through the
specialty shops or auction sites. But there's enough hard to find
titles here that would make searching these out worth it, if you don't
mind the bootleg quality and lack of liner note information.