this is from my experience(which goes far back so it might have changed a bit):
Summer Wing:
High Flier(easiest to come by with good or mint condition to a fair price)
Lady Flutter( almost as easy to find as Little flitter, but often comes in better condition, that is why she is before Little Flitter in the list)
Little Flitter( Easy to find, but hard to find with still pink hair and silky hair)
Sky Dancer( Medium hard to find, often find with frizzy hair and might have some discolor on her body too)
Glow( Medium hard to find, but same problem as with little flitter, hard to find with pink hair... has often faided to white)
Buzzer (Was the last one for me to find, so I guess he is medium hard to find...)
Windy Wing:
(All windy wings are more or less hard to find, but they also have a list)
Sunglider(according to me the eaiest to find among the WW)
Flurry( at the time I bouth her she was all over the place, but it was rare to find one with still pink hair.... so my guess is that this one is pending up and down)
Cool Breeze( if you have the money you could probably get a hold of this one... but she is prizy!)
Whirly and Moonjumper(these two are my twins... I had to look long and hard to find these two... and I found them almoust at the same time... I guess I was very lucky then)
Starry Wings(the hardest to find ever.... I looked for her for YEARS! VERY hard to find)
Difference between Summer Wings and Windy wings are as you said, their symbols and that SW has a forelock and WW does not....