Here is the difference between the two and you will see what I mean:
|Drawing courtesy Bluebird/Photobucket|
Sickling the foot usually arises from a dancer attempting to point her foot so hard that she curves the ankle inward toward her knee. Remember, the pointed foot comes from the stretch of the muscles over the top of the arch and on the sole of the foot, not the muscles on the sides of the foot. It may also occur when a dancer is pulling her foot to retire and attempting to keep the foot very close to the shin as she lifts it. The side of the ankle ends up touching the leg, rather than the toes.
When you sickle your foot, you are not only destroying the line of the straight leg and foot, but also training your ankle muscles to supinate which places too much weight on the outside of the ankle and can lead to sprained ankles.
Conversely, as we see above, winging is going in the opposite direction: pointing the foot toward the outside of the knee. This only looks good on stage! A winged foot when you do fouette turns, for example, will actually shorten the line when you fondu. In this case as well, you are training the muscles in an unstable position, placing too much weight on the big toe when you releve and again, this will lead to turned ankles.
So to sum up:
The good: a winged foot in arabesque on stage.
The bad: a winged foot en releve.
The ugly: a sickled foot at any time.