When I introduce the drop, If the kids is real "wristy"--has a quick release with wrist and fingers, I try the turnover first, those with slower releases and push the ball more I start with the peel.
Sometimes I have to change them, but generally it works out pretty good this way.
DD was never able to throw an effective drop with the peel. When she threw the pitch it was nothing more than a fastball with a greater downward slope. Generated very few strikeouts but was very effective at generating ground balls. With the turnover she gets a very significant drop at the end of the pitch. From the batters perspective the turnover drop starts off looking like a waist high fastball. By the end of the pitch it is almost in the dirt. Very effective and generates significant number of strikeouts BUT it also requires a catcher who can effectively drop and block otherwise you have a lot of dropped 3rd strike situations. One additional word of advice. Like other posters have said the peel was quite easy to learn. The turnover drop was not. It has taken her a year to throw effectively and she still has a ways to go before I would consider the pitch as consistent and effective as her fastball, change up or rise.
Peel drop is much easier to learn and teach, with much less chance of injury. Roll-over can generate more spin for girls who lack finger strength. Roll-over is telegraphed often by shorter stride and higher release though. Both can easily be made into drop-curves if that is the desired outcome.
Cat uses the peel, seems to work for her, let me know when you hear about a roll-over pitcher who's as successful.
My DD's PC taught her the turnover drop. She is 12 and it is an amazing pitch for her. She has good speed and the drop off is phenomenal. Her TB coach loves it.
Watch that video a few times. Tell me that her hand rolls over the ball to achieve spin. That's a peel drop with an IR followthrough very similar to Uneo.